The latest and greatest tricks in studying missing energy events

1. The latest and greatest tricks in studying missing energy events Konstantin Matchev With: M. Burns, P. Konar, K. Kong, F. Moortgat, L. Pape, M. Park arXiv:0808.2472 [hep-ph], arXiv:0810.5576 [hep-ph], arXiv:0812.1042 [hep-ph], arXiv:0903.4371 [hep-ph], arXiv:0906.2417 [hep-ph], arXiv:090?.???? [hep-ph] Fermilab, LPC August 10-14, 2009

2. 67 pp 46 pp 32 pp 47 pp 37 pp Total No of pages : 229 pp These slides cover: • “A general method formodel-independentmeasurements of particle spins, couplings and mixing angles in cascade decays with missing energy at hadron colliders”, JHEP (2008) • Burns, Kong, KM, Park • “Using subsystem MT2 for complete mass determinations in decay chains with missing energy at hadron colliders”, JHEP (2009) • Burns, Kong, KM, Park • “s1/2min – a global inclusive variable for determining the mass scale of new physics in events with missing energy at hadron colliders”, JHEP (2009). • Konar, Kong, KM • “Using kinematic boundary lines for particle mass measurements and disambiguation in SUSY-like events with missing energy”, JHEP (2009) • Burns, KM, Park • “Precise reconstruction of sparticle masses without ambiguities”, JHEP (200?) • KM, Moortgat, Pape, Park

3. MET events: experimentalist’s view • What is going on here? This is why I am interested in MET!

4. e e b n W t W n W n W n t e b e Why MET signatures are important to study • WIMP dark matter? Perhaps, but see J. Feng’s talk for counterexamples. • Challenging – need to understand the detector very well. • Guaranteed physics in the early LHC (late Tevatron) data!

5. The experimentalist asks: The theorist answers: Is it possible to have a theory model which gives signature X? Yes. No. Are there any well motivated such models? You bet. Let me tell you about those. Actually I have a paper… No. But I’m the wrong person to ask anyway. Is there any Monte Carlo which can simulate those models? MC4BSM workshops: http://theory.fnal.gov/mc4bsm/ This talk is being given • by a “theorist”

6. MET events: theorist’s view • Pair production of new particles (conserved R, KK, T parity) • Motivated by dark matter + SUSY, UED, LHT • How do you tell the difference? (Cheng, KM, Schmaltz 2002) • SM particles xi seen in the detector, originate from two chains • How well can I identify the two chains? Should I even try? • What about ISR jets versus jets from particle decays? • “WIMPs” X0 are invisible, momenta unknown, except pT sum • How well can I reconstruct the WIMP momenta? Should I even try? • What about SM neutrinos among the xi’s?

7. In place of a summary pessimism optimism pessimism optimism

8. MET Tuesday: invariant mass studies Hinchliffe et al. 1997 • Study the invariant mass distributions of the visible particles on one side of the event • Does not rely on the MET measurement • Can be applied to asymmetric events, e.g. • No visible SM products on the other side • Small leptonic BR on the other side • Well tested, will be done anyway. ATLAS TDR 1999 Nojiri et al. 2000 Allanach et al. 2000 Gjelsten et al. 2004 KM,Moortgat,Pape,Park 2009

9. Thursday: spin measurements Burns, Kong, KM, Park 08 • Separate the spin dependence from all the rest • Parameterize conveniently the effect from “all the rest” • Measure both the spin (S) as well as all the rest:

10. In place of a summary pessimism optimism pessimism optimism

11. Wednesday: Meff (HT) and Smin F. Paige hep-ph/9609373 Konar, Kong, KM 2008

12. In place of a summary pessimism optimism pessimism optimism

13. The “Cambridge” mT2 variable • A. Barr, C. Lester and P. Stephens, “mT2 : the truth behind the glamour” • hep-ph/0304226 • C. Lester and D. Summers, “Measuring masses of semiinvisibly decaying particles pair produced at hadron colliders” • hep-ph/9906349

14. Mass measurements • Single semi-invisibly decaying particle e W n • Use the transverse mass distribution

15. Mass measurements • A pair of semi-invisibly decaying particles e Lester,Summers 99 Barr,Lester,Stephens 03 W n n W m • Use the “stransverse” mass (mT2) Kong, KM 04 • This formula is valid for mn=0.

16. Definition of MT2 e Lester,Summers 99 Barr,Lester,Stephens 03 W n n W m A pair of semi-invisibly decaying particles If and were known: But since unknown, the best one can do :

17. What is mT2 good for? • So what? We still don’t know exactly the LSP mass • Provides a relation between the two unknown masses of the parent (slepton) and child (LSP) • Vary the child (LSP) mass, read the endpoint of mT2

18. LSP mass measurement from kinks Varying PT ISR with some PT • A kink appears at the true masses of the parent and the child Include pT recoil due to ISR

19. How big is this kink? FR FL It depends on the hardness of the ISR and the mass spectra

20. Origin of the MT2 “kink” FR • A kink may arise due to • “Composite” particle on each side • ISR recoils • Heavy particle decays FL Cho, Choi, Kim, Park 2007 Barr, Gripaios, Lester 2007 Burns, Kong, KM, Park 2008

21. Subsystem MT2 Burns, Kong, KM, Park 2008 • Generalize the MT2 concept to MT2(n,p,c) • “Grandparents” (n): The total length of decay chain • “Parents” (p): Starting point of MT2 analysis • “Children” (c): End point of MT2 analysis

22. Mass determination: Subsystem MT2 Burns, Kong, KM, Park 2008 Sub MT2 NP : Number of unknownsNm : Number of measurements NP= number of BSM particles = n+1 Nm= How many undetermined parameters (masses) are left? n : Length of decay chain

23. Opening a parenthetical remark

24. In place of a summary pessimism optimism pessimism optimism

25. Mass determination – polynomial method Cheng,Gunion, Han,Marandella, McElrath, 2007 Sub MT2 n : Length of decay chain

26. Closing the remark

27. Subsystem MT2 applied to top pairs MT2(220) e b t W n n W t b e Don’t assume prior knowledge of the W and neutrino masses Traditional MT2 variable: MT2(2,2,0) Combinatorial problem!

28. Subsystem MT2 applied to top pairs MT2(210) e b t W n n W t b e No combinatorial problem! Genuine subsystem variable: MT2(2,1,0)

29. Subsystem MT2 applied to top pairs MT2(221) e b t W n n W t b e No combinatorial problem! Another genuine subsystem variable: MT2(2,2,1)

30. Mass measurements in the TTbar system • We have just measured three MT2 endpoints which are known functions of the hypothesized Top, W and neutrino masses. • MT2(2,2,0) • MT2(2,1,0) • MT2(2,2,1) • Problem: they are not independent, need an additional measurement • MT2(1,1,0) • Endpoint of the lepton+b-jet inv. mass distribution

31. MT2 applied to W pairs e W n W n e MT2(110) No combinatorial problem! Yet another MT2 variable: MT2(1,1,0)

32. Full T, W, Nu mass determination M(bl)max = b e t W n Correct bl pairs W n t b e Hybrid method: Inv. mass Subsystem MT2

33. e e b b LSP t W n stop chargino stop chargino LSP W n t b e b e On a positive note Barr, Gwenlan 2009 • MT2 can be used for background suppression • The dominant background to SUSY is TTbar • For illustration, let us choose a very challenging example with an identical signature • Stop pair production, with decays to chargino and LSP.

34. Top-Stop separation • What do we know about the stop sample? • Absolutely nothing. • What do we know about TTbar? • The endpoints of the subsystem MT2 variables that we just saw. All TTbar events fall below these endpoints, and there are none above! KM, Park preliminary

35. Combination MT2 cut • Accept the event if it is beyond at least one of the three subsystem MT2 endpoints. • This greatly enhances the signal acceptance, compared to a single MT2 cut, or an HT cut.

36. BACKUPS

37. Wedgebox technique • Scatter plot of the invariant masses of the visible decay products on both sides Bisset,Kersting,Li,Moortgat,Moretti,Xie 2005

38. MTgen Lester,Barr 2008 • Inclusive application of MT2: minimize MT2 over all possible partitions of the visible decay products between two chains • Brute force way to deal with combinatorial issue • Preserves the endpoint, provides a measure of the scale • Endpoint smeared in the presence of ISR • Does not measure the LSP mass • Difficult to interpret when many processes contribute

42. Polynomial method Cheng,Gunion,Han,Marandella,McElrath 2007 Cheng,Engelhardt,Gunion,Han,McElrath 2007 Cheng,Han 2008