Beyond the Standard Model Physics

1. Beyond the Standard Model Physics Peter Richardson IPPP, Durham University and CERN Theory Group CTEQ 13th August

2. Outline • Yesterday • Why BSM Physics? • Where will we look for it? • What are the models • Today • Collider Signatures • Discovery channels • Determining the model • Conclusions CTEQ 13th August

3. Generic BSM signatures • Before we go on and consider the signals of models of new physics in great detail it is worthwhile considering what we expect to see in general. • Most models of new physics predict either: • the existence of more particles than the Standard Model; • new operators which give deviations from the Standard Model predictions. • The signatures of the model depends on either: • how these particles are produced and decay; • the type of deviations expected. CTEQ 13th August

4. Backgrounds • So I’m going to exclusively talk about signals, however getting the background right is essential. • In fact really for BSM physics the most important things at this school have been said by the other lecturers. • It’s important that the Standard Model processes and detectors are under control and that we have accurate Standard Model simulations. CTEQ 13th August

5. Deviations from the Standard Model • So there can be deviations from what is expected in the Standard Model due to: • due to compositeness; • exchanging towers of Kaluza-Klein gravitons in large extra dimension models; • unparticle exchange; • … • Tends to give changes in the shapes of spectra. • Therefore in order to see a difference you need to know the shape of the Standard Model prediction. CTEQ 13th August

6. Example I: High-pT jets • One possible signal of compositeness is the production of high pT jets. • At one point there was a disagreement between theory and experiment at the Tevatron. • Not new physics but too little high-x gluon in the PDFs. CTEQ 13th August

7. Example II: Unparticles • Many models predict deviations in the Drell-Yan mass spectra. • For example in an unparticle model with the exchange of virtual spin-1 unparticles. Cheung et. al. Phys.Rev.D76:055003,2007. CTEQ 13th August

8. Monojets • In some models either stable neutral particles can be produced recoiling against a jet. • Or a tower of KK gravitons in large extra dimension models. • Or unparticles. • … • Gives a monojet signal. CTEQ 13th August

9. Monojets • Many Standard Model electroweak backgrounds • CDF results taken from Fermilab wine and cheese seminar by K. Burkett. CTEQ 13th August

10. New Particle Production • In general there are two cases for models in which new particles are produced • The model has only a few new particles, often mainly produced as s-channel resonances: • Z-prime models; • Little Higgs; • Small extra dimensions. • The model has a large number of new particles: • SUSY; • UED; • Little Higgs with T-parity. CTEQ 13th August

11. Resonance Production • The easiest and cleanest signal in hadron collisions is the production of an s-channel resonance which decays to e+e- or m+m-. • Resonances in this and other channels are possible in: • Little Higgs models; • Z’ models; • UED; • Small Extra Dimensions. CTEQ 13th August

12. Example: Resonant Graviton Production • The best channel, e+e-, gives a reach of order 2 TeV depending on the cross section. • Other channels m+m-, gg, WW are possible. • If light enough the angular distribution of the decay products can be used to measure the spin of the resonance. Allanach et. al. JHEP 0009:019, 2000 CTEQ 13th August

13. Hadronic Resonances • A lot of models predict hadronic resonances. • Much more problematic due to the mass resolution. • Smears out narrow resonances and the QCD backgrounds are often huge. • Can do background subtraction but dealing with huge S/B ratios. CTEQ 13th August

14. SUSY-like models • Most of the other models are ‘SUSY’-like, i.e. they contain: • a partner of some kind for every Standard Model particle; • often some additional particles such as extra Higgs bosons; • a lightest new particle which is stable and a dark matter candidate. • As these are the most popular models I’ll spend most time on them. CTEQ 13th August

15. New Particle Production • A lot of new particles should be produced in these models. • While some particles may be stable, i.e. the decay length of the particle is such that the majority of the particles escape from the detector before decaying. In practice this happens for lifetimes greater than 10-7s. • However the majority of these particles decay to Standard Model particles. CTEQ 13th August

16. New Particle Production • Therefore we expect to see: • charged leptons; • missing transverse energy from stable neutral particles or neutrinos; • jet from quarks, perhaps with heavy bottom and charm quarks; • tau leptons; • Higgs production; • photons; • stable charged particles. • It’s worth noting that seeing an excess of these doesn’t necessarily tell us which model we’ve seen. CTEQ 13th August

17. New Particle Production • The archetypal model containing large numbers of new particles which may be accessible at the LHC is SUSY. • Other models are • UED • Little Higgs with T-parity • However in practice UED is mainly used as a straw-man model for studies trying to show that a potential excess is SUSY. CTEQ 13th August

18. The LHC • Two statements which are commonly made are: • The LHC will discover the Higgs; • The LHC will discover low-energy SUSY if it exists. • The first is almost certainly true. • The second is only partially true. CTEQ 13th August

19. SUSY Production • In hadron collisions the strongly interacting particles are dominantly produced. • Therefore in SUSY squark and gluino production has the highest cross section. CTEQ 13th August

20. SUSY Decays • These particles then decay in a number of ways. • Some of them have strong decays to other strongly interacting SUSY particles. • However the lightest squark/gluino can only decay weakly. • The gluino can only have weak decays with virtual squarks or via loop diagrams. CTEQ 13th August

21. SUSY Decays • This is the main production mechanism for the weakly interacting SUSY particles. • The decays of the squarks and gluinos will produce lots of quarks and antiquarks. • The weakly interacting SUSY particles will then decay giving more quarks and leptons. CTEQ 13th August

22. SUSY Decays • Eventually the lightest SUSY particle which is stable will be produced. • This behaves like a neutrino and gives missing transverse energy. • So the signal for SUSY is large numbers of jets and leptons with missing transverse energy. • This could however be the signal for many models containing new heavy particles. CTEQ 13th August

23. SUSY Searches • All SUSY studies fall into two categories • Search Studies • Designed to show SUSY can be discovered by looking a inclusive signatures and counting events. • Measurement Studies • Designed to show that some parameters of the model, usually masses, can be measured. CTEQ 13th August

24. General SUSY Signals • There’s a large reach looking for a number of high transverse momentum, pT, jets and missing transverse energy. Taken from the CMS Physics TDR CTEQ 13th August

25. General SUSY Signals • Or with additional leptons as well. Taken from the CMS Physics TDR CTEQ 13th August

26. General SUSY signals • Also possible to have the production of Z and Higgs bosons and top quarks. • In many cases the tau lepton may be produced more often than the electron and muon. CTEQ 13th August

27. Measuring the Mass Scale • For events which have at least four jets and missing transverse energy. • The scale can be estimated using • This variable is strongly correlated with the mass of strongly interacting SUSY particles CTEQ 13th August

28. Measuring the Mass Scale • Can measure the squark/gluino mass to about 15%. • Taken from Phys.Lett. B498, 1 (2001.), D. Tovey. CTEQ 13th August

29. Discovering SUSY • The analyses we have just looked at are those that are used to claim the LHC will discover SUSY. • Is that what they tell us? • They don’t really discover SUSY. • What they see is the production of massive strongly interacting particles. • Doesn’t have to be SUSY, could be something else. CTEQ 13th August

30. Discovering SUSY • In order to claim that a signal is SUSY we would need to know more about it. • SUSY analyses tend to proceed by looking for characteristic decay chains and using these to measure the masses of the SUSY particles. CTEQ 13th August

31. SUSY Decay chains • The two most common chains are • Higgs production, • Lepton production, , perhaps via an intermediate slepton. • These are used as a starting point for mass measurements. • The masses of other particles can be measured by adding more leptons or jets and working up the decay chain. CTEQ 13th August

32. SUSY Mass Reconstruction • In many models the decay chain • exists. • The end-point of the dilepton mass distribution gives the mass difference of the neutralinos. • Use the leptons to measure the mass difference of the neutralinos. CTEQ 13th August

33. SUSY Mass Reconstruction • Most of these studies are essentially an exercise in relativistic kinematics. • For example if the decay is three body the endpoint is • If the decay has an intermediate slepton the endpoint is CTEQ 13th August

34. Deriving the end-points • These end-point results are always easiest to calculate in a specific frame. In this case the rest frame of the slepton is best. • In this frame the lepton frame the lepton from the neutralino decay has momentum. • The end-point occurs when the lepton produced in the decay is back-to-back with this • Summing the 4-momenta gives the end-point. CTEQ 13th August

35. SUSY Mass Reconstruction • Other information can be obtained by adding in more leptons and jets. • By adding a jet to this can measure more end-points in the lepton-q and lqq distributions. • This provides enough kinematic end-points to measure all the masses in the decay chain. • Most recent studies have made use of this. CTEQ 13th August

36. SUSY Mass Reconstruction • The mass of the LSP here is obtained to 14%, the slepton to 9%, the second neutralino to 8% and the squark to 5%. • The small errors on the heavier states are due to their larger masses. The dominant error is the overall scale. • Taken from Allanach et.al. JHEP 0009:004,2000. CTEQ 13th August

37. Other Masses • It is also possible to reconstruct the gluino mass. • In some cases the heavier neutralino and chargino masses can be measured. • This all seems very promising. • Many masses can be measured and we have learnt a lot. CTEQ 13th August

38. Problems • Would this enough to say we have discovered supersymmetry? • It strongly depends on specific decay chains being present. • Also in simulations we know what model was put in, unless someone has divine inspiration this won’t happen with data. • It’s not clear how this will effect things. There are a lot of hidden assumptions. • Doing a general analysis without these is very hard. • However once we seen something things may be clearer. CTEQ 13th August

39. Problems • Equally analysis techniques have changed a lot in recent years. • The top mass analysis of 10 years ago was very different to the approaches currently used. • There’s no telling what analysis techniques will be developed or applied to BSM signals during LHC running. CTEQ 13th August

40. Dark Matter • One of the hottest topics for some time has been the relation between cosmology, in particular dark matter, and collider experiments. • One aspect of this is using the relic density to constrain the SUSY parameter space. • Another is the idea that if we see SUSY this would tell us the mass of the dark matter and something about its interactions. CTEQ 13th August

41. Constraints • The most common additional assumptions are that the model must satisfy: • cosmological constraints from WMAP; • bgsg constraint; • g-2 constraint. • Opinions on how much weight to give to these constraints varies. CTEQ 13th August

42. Cosmological Constraints • Brown has a charged LSP. • Pink favoured by g-2. • Green excluded by b to sg • Cyan favoured by older cosmological constraints. • Blue by the WMAP results. Taken from Phys.Lett.B565, 176 Ellis et.al. CTEQ 13th August

43. Cosmological Constraints • Some people would argue that we want the model to be consistent with all the limits. • Others, myself included, would argue that many of the things constrained by these limits do not have a major impact on the collider signals. • Often they depend on masses of say the heavier Higgs in (co)-annihilation diagrams which does not have a major effect on the collider signals. CTEQ 13th August

44. Measuring BSM Properties • If its there the LHC should see TeV scale physics in a number of models. • Once we have seen evidence of such physics we will need more accurate measurements to determine what it is. CTEQ 13th August

45. q e- q e+ Measuring the Spin • Due to the couplings there are significant spin correlations in the quark-lepton mass distribution. • This is best seen by looking at the end-point of the mass distribution. • This configuration is favoured if the neutralino decays to a positive lepton and disfavoured for a negative lepton. Spin 1 Spin 0 CTEQ 13th August

46. Measuring the Spin • The problem is that this asymmetry vanishes if we can’t distinguish between quarks and antiquarks. • However the LHC produces more squarks than antisquarks because there are more incoming quarks. • Gives a measurable charge asymmetry CTEQ 13th August

47. Measuring the Spin With spin correlations Without spin correlations Taken from Barr hep-ph/0405052 CTEQ 13th August

48. Measuring the Spin • A similar decay chain exists in the UED model. • Used as a straw-man for SUSY. • The particle spins are different, as are the couplings, leads to different correlations. • Taken from Smillie and Webber JHEP 0608:055,2006 Red UED, dashed SUSY, dotted phase space CTEQ 13th August

49. SUSY Variants • So I’ve talked a lot about the MSSM, or in reality the SUGRA model. • There are variants of the SUSY model which are interesting: • GMSB can give a stau which is stable on collider time-scales; • Split SUSY models in which the scalars are very heavy give interesting signals; • R-parity models in which the LSP decays. CTEQ 13th August

50. Split SUSY • In the Split SUSY model the scalars are very heavy. • Leads to a gluino which is stable on collider timescales. • The gluino hadronizes giving heavy hadrons which contain the gluino. • When the gluino hadronizes it will form either • Glueball-like state • Mesonic state • Baryonic state • General opinion is that • Rg is the lightest state • Rqqq is unlikely to be directly produced. CTEQ 13th August