Measuring new particle masses with M T2 at the LHC (I)

1. Measuring new particle masses with MT2 at the LHC (I) Yeong Gyun Kim (KAIST) APCTP 2008 LHC Physics Workshop at Korea, 19-21 August, 2008

2. PR06.0807.08.2008 CERN announces start-up date for LHC Geneva, 7 August 2008, CERN has today announced that the first attempt to circulate beam in the Large Hadron Collider (LHC) will be made on 10 September. This news comes as the cool down phase of commissioning CERN’s new particle accelerator reaches a successful conclusion. Television coverage of the start-up will be made available through Eurovision. The CERN Control Center, from where the LHC be operated

3. We are entering exciting period in particle physics. The LHC is about to explore for the first time the TeV energy scale. The origin of EWSB ? The nature of dark matter ? Supersymmetry ? Extra dimensions ?

4. General features for SUSY at the LHC • SUSY production is dominated by gluinos and squarks, unless they are too heavy • The gluinos and squarks cascade down, generally in several steps, to the final states including multi-jets (and/or leptons) and invisible two LSPs Multi-Jets + Missing ET (+ N leptons etc.)

5. Discovery potential 5s evidence after 1 fb-1 (including systematics) expected if squarks lighter than 1300 GeV 0-lepton and 1-lepton best modes for mSUGRA No attempt to combine channels yet preliminary ~ ~ (Taken from T.Lari’s talk in LHC focus week at IPMU) T. Lari INFN Milano

6. Discovery of Supersymmetry Mass measurements Measurements of spin, couplings

7. Why SUSY mass measurement is Not an easy task at the LHC ? • Final state momentum in beam direction is unknown a priori, due to our ignorance of initial partonic center of mass frame • SUSY events always contain two invisible LSPs •  No masses can be reconstructed directly

8. Several approaches (and variants) of mass measurements proposed • Invariant massEdge method Hinchliffe, Paige, Shapiro, Soderqvist, Yao ; Allanach, Lester, Parker, Webber • Mass relation method Kawagoe, Nojiri, Polesello ; Cheng, Gunion, Han, Marandellea, McElrath • Transverse mass(MT2 ) kink method Cho, Choi, YGK, Park ; Barr, Lester, Gripaios ; Ross, Serna; Nojiri, Shimizu, Okada, Kawagoe

9. Invariant mass edge method Hinchliffe, Paige, etal. (1997) • Basic idea  Identify a particular long decay chain and measure kinematic endpoints of various invariant mass distributions with visible particles  The endpoints are given by functions of SUSY particle masses

10. If a long enough decay chain is identified, It would be possible to measure sparticle masses in a model independent way 3 step two-body decays

11. Invariant mass edges In total, five endpoint measurements Four invovled sparticle masses can be obtained for SPS1a point

12. Consider the following cascade decay chain • (4 step two-body decays) Mass relation method Kawagoe, Nojiri, Polesello (2004) • Completely solve the kinematics of the cascade decay • by using mass shell conditions of the sparticles

13. One can write five mass shell conditions which contain 4 unknown d.o.f of LSP momentum •  Each event describes a 4-dim. hypersurface • in 5-dim. mass space, and the hypersurfcae • differs event by event • Many events determine a solution for masses through intersections of hypersurfaces

14. Measurements of gluino and sbottom masses • (assuming that the masses of two neutralinos and • slepton are already known) in SPS 1a point Kawagoe, Nojiri, Polesello (2004) In this case, each event corresponds to a different line in plane Two events are enough to solve the gluino and sbottom masses altogether Build all possible event pairs (with some conditions) m_gluino ~ 592 GeV (300 fb-1) Gluino mass distribution with event pair analysis

15. Both of the Edge method and the Mass relation method • rely on a long decay chain to determine sparticle masses • What if we don’t have long enough decay chain but only short one ? • In such case, MT2 variable would be useful to get information on sparticle masses

16. For the decay, ( ) • Cambridge mT2 (Lester and Summers, 1999) Massive particles pair produced Each decays to one visible and one invisible particle. For example,

17. ( : total MET vector in the event ) However, not knowing the form of the MET vector splitting, the best we can say is that : with minimization over all possible trial LSP momenta

18. ( with ) • MT2 distribution for LHC point 5, with 30 fb-1, Endpoint measurement of mT2 distribution determines the mother particle mass (Lester and Summers, 1999)

19. MT2 distribution over many events: (Taken from Lester’s talk in the LHC focus week at IPMU) mT2(mB) mB mA

20. Varying “”… (Taken from Lester’s talk in the LHC focus week at IPMU) Does not just translate … Shape may also change … more on this later. mT2() mB mA

21. Actual change in evidence on a log plot (Taken from Lester’s talk in the LHC focus week at IPMU) arXiv: 0711.4008 mT()

22. (Lester and Summers, 1999) The LSP mass is needed as an input for mT2 calculation But it might not be known in advance mT2 depends on a trial LSP mass Maximum of mT2 as a function of the trial LSP mass The correlation from a numerical calculation can be expressed by an analytic formula in terms of true sparticle masses

23. The maximum of the squark mT2as a function of m_chi (Cho, Choi, YGK and Park, 2007) Well described by the above Analytic expression with true Squark mass and true LSP mass • Squark and LSP masses are • Not determined separately

24. We considered ‘Squark’ mT2 • Now consider ‘Gluino’ mT2 (Cho, Choi, YGK and Park, 2007)

25. Each mother particle produces one invisible LSP and more than one visible particles Possible mqq values for three body decays of the gluino :

26. MT2 maximum as a function of trial LSP mass • depends on di-quark invariant mass (mqq) mqq=maximum MT2 maximum mqq=mqq mqq=minimum Trial LSP mass (For simplicity, here we assume mqq (1) = mqq (2) )

27. Experimental feasibility An example (a point in mAMSB) with a few TeV sfermion masses (gluino undergoes three body decay) Wino LSP We have generated a MC sample of SUSY events, which corresponds to 300 fb-1by PYTHIA The generated events further processed with PGS detector simulation, which approximates an ATLAS or CMS-like detector

28. The gluino mT2 distribution with the trial LSP mass mx = 90 GeV Fitting with a linear function with a linear background, We get the endpoints mT2 (max) = The blue histogram : SM background

29. as a function of the trial LSP mass • for a benchmark point GeV The true values are Fitting the data points with the above two theoretical curves, we obtain

30. m2qq max = Therefore, • For case of two body cascade decay ,

31. For three body decay For two body cascade decay

32. 6-quark mT2(I) If m_squark > m_gluino and squark is not decoupled, Squark  quark + gluino ( q q LSP)  3-quarks + LSP Maximum of the Invariant mass of 3-quarks M_qqq (max) = m_squark – m_LSP , if (m_gluino)^2 > (m_squark * m_LSP)

33. 6-quark mT2 (II) A mSUGRA point, m_squark ~ 791 GeV, m_gluino ~ 636 GeV, m_LSP ~ 98 GeV PT (7th-jet) <50 GeV Hemishpere analysis

34. 6-quark mT2 (III)

35. where • In principle, we don’t have to identify particular chain, if we can measure visible invariant mass range experimentally. If we know minimum and maximum of the visible invariant mass for mother particle decay, we can use two theoretical curves to identify kink position.

36. (Cho,Choi, YGK, Park, arXiv:0804.2185) Standard Candle for MT2 study Top quark mT2 distribution with m_nu = 0

37. mT2 max vs. trial neutrino mass Shape of mT2 distribution Standard Candle for MT2 study The di-leptonic channel will provide a good playground for mT2 excercise

38. M2C (A Variant of ‘gluino’ mT2) • Ross and Serna (arXiv:0712.0943 [hep-ph]) A Variant of ‘gluino’ mT2 with explicit constraint from the endpoint of ‘diquark’ invariant mass (M2C) • MTGen vs. Hemisphere analysis • Barr, Gripaios and Lester (arXiv:0711.4008 [hep-ph]) Instead of jet-paring with hemisphere analysis, we may calculate mT2 for all possible divisions of a given event into two sets, and then minimize mT2

39. Inclusive mT2 • Nojiri, Shimizu, Okada and Kawagoe (arXiv:0802.2412) Even without specifying the decay channel, mT2 variable still shows a kink structure in some cases. This might help to determine the sparticle masses at the early stage of the LHC experiment

40. The Horror of the Heights (1913) Arther Conan Doyle

41. NOTHING … in the TeV scale • The Horror of the Heights (2008-)

42. The Hope of the Heights (2008-) SUSY jungle (sps1a) “ There are jungles of the upper air, and there are worse things than tigers which inhabit them. I believe in time they will map these jungles accurately out “ - Armstrong in ‘the Horror of the Height’