1. BSM YETI Meeting 10 th Jan SUSY Decays Peter Richardson IPPP, Durham

2. BSM YETI Meeting 10 th Jan Plan In this talk I will recap a few basic ideas in collider phenomenology. The aim is to allow you to understand the following –What particles are produced? –How to they decay? –Which decays give interesting signals? We will follow this with a practical looking at some different models and their signatures.

3. BSM YETI Meeting 10 th Jan Collider Phenomenology Given the LHC will start in less than two years I will concentrate on hadron collider phenomenology in most of what I say. Hadron Collider phenomenology is essential the phenomenology of QCD. The incoming quarks and gluons inside the protons interact via the strong force and therefore processes mediated by the strong force will be the most important.

4. BSM YETI Meeting 10 th Jan A Typical Hadron Collider Event

5. BSM YETI Meeting 10 th Jan A Tevatron Event

6. BSM YETI Meeting 10 th Jan Generic BSM signatures Before we go on and consider models of new physics in great detail it is worthwhile considering what we expect to see in general. Most models of new physics predict the existence of more particles than the Standard Model. We hope that these will be produced in collider experiments. The signature of the model then depends on how these particles are produced and decay.

7. BSM YETI Meeting 10 th Jan Generic BSM signatures One scenario is that the particle is stable. By stable I mean that 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 -7 s. The other option is that the particle decays. I don’t know of a model in which these decays do not to produce Standard Model particles.

8. BSM YETI Meeting 10 th Jan Generic BSM signatures Therefore we expect to see Charged leptons (for experimentalists this means electrons and muons) Missing transverse energy from stable particles or neutrinos Jet from quarks, perhaps with heavy bottom and charm quarks Tau leptons. Higgs production Photons It’s worth noting that seeing an excess of these doesn’t necessarily tell us what we’ve seen.

9. BSM YETI Meeting 10 th Jan Tevatron Top Events Before we go on and look at signal for BSM physics it is worthwhile studying the top quark signal from the Tevatron. I will go through this fairly slowly to give you an idea of the steps to go through and what needs to be thought about when analysing a new physics signal. After all before its discovery the top was new physics as well.

10. BSM YETI Meeting 10 th Jan Top Events Let’s start by thinking about how the top quark is produced and decays? The top quark is produced by the strong interaction. In general if a strong process is possible in hadron collisions it will be the most important due to the large coupling constant.

11. BSM YETI Meeting 10 th Jan Top Events The top has a very short lifetime and decays. Therefore the next thing to consider is the decay. What does the top decay to? Does it have a strong decay? Does it have a weak decay? No Yes

12. BSM YETI Meeting 10 th Jan Top Events

13. BSM YETI Meeting 10 th Jan Top Events The W is unstable and also decays. The W can decay to either a quark and an antiquark or a lepton and a neutrino. Therefore the possible signals of top production are –Two b quarks, two leptons and two neutrinos –Two b quarks, two other quarks a lepton and a neutrino –Two b quarks and four other quarks The quarks will give jets in the detector.

14. BSM YETI Meeting 10 th Jan Top Events The W bosons can also decay to give quarks.

15. BSM YETI Meeting 10 th Jan Top Signals There are two things to consider when deciding which one of these signals is best. The backgrounds for particles which can only be produced by the electroweak force are smaller. Pure QCD backgrounds are the largest. We want to be able to reconstruct the top quark mass. This means we need the momenta of the decay products. There is a balance between these two aims.

16. BSM YETI Meeting 10 th Jan Top Signals For the six jet signal the backgrounds are high but we can reconstruct the top mass, although there are combinatoric problems. In hadron collisions we don’t know how much of the beam energy went into the hard collision. Therefore we can only reconstructed the missing transverse momentum using momentum conservation. For each neutrino there are three unknown momentum components (if we assume they are massless.)

17. BSM YETI Meeting 10 th Jan Top Signals Conservation of transverse momentum gives two constraints. In the top signal we know the neutrino came from a W and therefore the W mass gives another constraint. Therefore we can reconstruct events with one missing neutrino but not two. Therefore the two charged lepton signal which has the lowest background isn’t so good.

18. BSM YETI Meeting 10 th Jan Top Signals The best signal for top events is where one W decays hadronically and the other leptonically. The signal is therefore –Two jets with displaced vertices from the b quarks. –Two jets which reconstruct to give a W boson. –A charged lepton. –A neutrino which gives missing transverse energy because it doesn’t interact in the detector.

19. BSM YETI Meeting 10 th Jan Tevatron Top Candidate Example of a Tevatron top candidate with one lepton.

20. BSM YETI Meeting 10 th Jan Top Events We have now looked at the first part of any hadron collider search for new physics, i.e. what the signal looks like. We know what we will see in the detector and what objects to combine to give the particles we are looking for. The second and often more important part is to work out what other processes could look like this. If we consider the process where one W decays to a charged lepton and a neutrino.

21. BSM YETI Meeting 10 th Jan Top Production with Leptonic W Decay We have two jets containing bottom quarks. Two jets from other quarks. A charged lepton and neutrino from W decay. The backgrounds are other processes with a W. The classic example is the Drell-Yan process.  The incoming particles can then radiate to give additional jets.

22. BSM YETI Meeting 10 th Jan Top production Here are the results from CDF at Fermilab looking at events with a lepton, missing energy and at least three jets. The lepton has transverse energy greater than 20 GeV. The missing transverse energy is greater than 20 GeV. The jets have transverse energy greater than 15 GeV. H T is the sum of the transverse energies of the lepton, jets and missing energy.

23. BSM YETI Meeting 10 th Jan Top Production This type of study is essentially a counting experiment. Events are selected using some experimental cuts. The number of events passing the cut is recorded, here as a function of H T. The number of background events is calculated and the signal fitted to agree with the data. This is similar to many of the general SUSY search strategies.

24. BSM YETI Meeting 10 th Jan Backgrounds and Monte Carlo In hadron collider physics the most important thing is to get the calculation of the background right. The calculation of the signal is usually much easier and less important. This is often overlooked by theorists altogether. Equally the answer from experimentalists is usually “well I ran PYTHIA.” Come to the Standard Model YETI in March to learn why this isn’t always the best idea.

25. BSM YETI Meeting 10 th Jan SUSY Models The MSSM has too many parameters to deal with in any experimental study. All studies use some high scale model to predict the MSSM parameters in terms of a few inputs. The three most commonly used are 1)SUGRA, supergravity mediated (CMSSM.) All scalar masses (M 0 ), unified at the GUT scale. All gaugino masses (M 1/2 ), unified at the GUT scale. Universal A and B terms. All masses predicted in terms of five parameters

26. BSM YETI Meeting 10 th Jan SUSY Models 2)GMSB, gauge mediated SUSY breaking transmitted by gauge fields. Gaugino masses occur at one-loop Scalar masses at two loop. Lightest SUSY particle (LSP) is the gravitino. Phenomenology depends on the NLSP. 3)AMSB, anomaly mediated Superconformal anomaly always present. Masses in terms of gravitino mass. Needs universal scalar mass term to avoid tachyonic particles.

27. BSM YETI Meeting 10 th Jan SUSY Spectrum The mass spectrum in different in the models. Gives different collider signals which are worth studying. Different splittings between the weakly and strongly interacting states. Features like the chargino neutralino splitting in AMSB

28. BSM YETI Meeting 10 th Jan SUSY Models These models help illustrate which models are worth studying. There are variants of the AMSB model which don’t predict the small mass difference between the lightest chargino and the LSP. As this is the only major qualative difference between AMSB and other models these models haven’t been studied in the same detail.

29. BSM YETI Meeting 10 th Jan SUSY Production In hadron collisions the strongly interacting particles are dominantly produced. Therefore in SUSY squark and gluino production has the highest cross section.

30. BSM YETI Meeting 10 th Jan 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.

31. BSM YETI Meeting 10 th Jan 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.

32. BSM YETI Meeting 10 th Jan 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.

33. BSM YETI Meeting 10 th Jan SUSY Searches All SUSY studies fall into two categories 1)Search Studies Designed to show SUSY can be discovered by looking a inclusive signatures and counting events. 2)Measurement Studies Designed to show that some parameters of the model, usually masses, can be measured.

34. BSM YETI Meeting 10 th Jan SUSY at the Tevatron Given the Tevatron is still running its worth briefly mentioning the signals there. Given the low energy the best signals tend to be the direct, electroweak, production of charginos and neutralinos gives the best signal. At the LHC the Standard Model background to this are huge and its hard to observe.

35. BSM YETI Meeting 10 th Jan SUSY at the Tevatron Taken from Dedes et. al hep-ph/0207026. Green is with 2fb -1 which is about what may now be achieved.

36. BSM YETI Meeting 10 th Jan General SUSY Signals There’s a large reach looking for a number of high transverse momentum, p T, jets and leptons and missing transverse energy.

37. BSM YETI Meeting 10 th Jan 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.

38. BSM YETI Meeting 10 th Jan 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

39. BSM YETI Meeting 10 th Jan 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.

40. BSM YETI Meeting 10 th Jan 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 analyses have made use of this.

41. BSM YETI Meeting 10 th Jan SUSY Mass Reconstruction Using these three edge, plus the dilepton edge gives four constraints on two neutralino, one slepton and squark masses. This is enough to reconstruct everything. The mass differences are well measured but the error on the overall scale, given by the LSP mass is 10-15%. lq massllq mass

42. BSM YETI Meeting 10 th Jan SUSY Mass Reconstruction In other models the decay modes Exist and can be used to work out the masses. In GMSB the neutralino decays and its decay products can be used to find the masses.

43. BSM YETI Meeting 10 th Jan Summary I’ve tried to give you an basic idea of the phenomenology of SUSY decays. Hopefully you now know enough to try looking at things for yourselves in the practical.