Preparing for the Early Years of the Large Hadron Collider

1. Preparing for the Early Years of the Large Hadron Collider  Overview Lecture Given at the 4th RTN Workshop Varna, Bulgaria Matthew J. Strassler Rutgers University (New Jersey, USA)

2. The Context The LHC: the best possible place to look for new physics the worst possible place to look for new physics • What makes it so horrible?! • How do we deal with the challenges • Jets are everywhere • What are jets anyway? • The Standard Model is a source of large backgrounds to most signals • What are the most important, and in what sense? • And now we want to find new physics • But what does it look like? • What kind of backgrounds must be understood? • What should we expect? What should we be careful of?

3. What I won’t talk about • Heavy Ion Collisions and the connection with String Theory • [see H. Liu’s talk] • Diffractive Higgs production and the connection with string theory • [Pomeron] – not early LHC • All those different models of • extra dimensions, • deconstructed extra dimensions, • theories dual to extra dimensions • fermionic extra dimensions (well, a few words) • Black Holes • BBC Reporting

4. LHC Cross sections vary over many orders of magnitude • Every aspect of the experiment is influenced by this graph • Note the ratios of • inelastic to b pairs • b pairs to W • W to top pairs • top pairs to Higgs • top pairs to TeV-scale SUSY • Higgs to Higgs  photons

5. If we kept all the events we’d have 1015 / year In fact 99.9999% must be instantly discarded The trigger is necessary, crucial, and introduces unavoidable bias Even after the trigger, ~109 events/year ~103 physicists An automated system must quickly analyze the huge amount of data from each event. Another necessary, crucial, and potentially biasing stage

6. Standard Model produces huge backgrounds: hard to calculate, or measure, or model Theory is way behind Experimentalists will determine many backgrounds by measuring This is not always possible and is fraught with dangerous assumptions Theory bias can creep in here as well.

7. ATLAS, CMS, LHCb, ALICE,... • General purpose detectors ATLAS, CMS our main focus • What can these detectors do? • What can’t they do?

8. Quick review: LHC Kinematics • Everything is oriented along (longitudinal) and perpendicular (transverse) to beam-pipe • The c.m. frame of proton-proton collision is the lab frame • But c.m. frame of scattering quarks/antiquarks/gluons is not the lab frame • Typical scattering is boosted along beampipe • therefore total energy, z-momentum not known

9. Quick review: LHC Kinematics 7 TeV 7 TeV • Everything is oriented along (longitudinal) and perpendicular (transverse) to beam-pipe • The c.m. frame of proton-proton collision is the lab frame • But c.m. frame of scattering quarks/antiquarks/gluons is not the lab frame • Typical scattering is boosted along beampipe • therefore total energy, z-momentum not known

10. Quick review: LHC Kinematics x1 7 TeV The scattering “partons” carry fractions x1, x2 of their protons momentum x2 7 TeV • Everything is oriented along (longitudinal) and perpendicular (transverse) to beam-pipe • The c.m. frame of proton-proton collision is the lab frame • But c.m. frame of scattering quarks/antiquarks/gluons is not the lab frame • Typical scattering is boosted along beampipe • therefore total energy, z-momentum not known

11. Quick review: LHC Kinematics • Can only apply conservation of transverse momentum • Energy and z-momentum of scattering partons not known • Not observable: destroyed proton debris lost down beampipe • Transverse momentum (2-d vector) : pT • NOT longitudinal momentum • NOT energy • Missing transverse momentum if SpT not zero. • Called “MET” or Missing (Transverse) Energy x1 7 TeV The scattering “partons” carry fractions x1, x2 of their protons momentum x2 7 TeV

12. What’s easy, what’s hard? • Relatively easy: • Detecting and measuring isolated electrons, muons, photons • Very hard • Measuring jet energies, momenta for jets (pT> 50 GeV) • Interpreting them as quark/gluon energies and momenta • Extremely hard • Detecting/measuring low-pT jets • Measuring missing transverse momentum • Virtually impossible • Telling quark jets from gluon jets or antiquark jets • Seeing electrons or photons inside of jets

13. What’s easy, what’s hard • Easy: Measuring cross-sections times branching ratios times efficiencies Number of events proportional to • Cross section for production , times • Branching fraction into particular final state , times • “Efficiency” for detecting the particular final state • Hard: • Measuring cross-sections times branching ratios • Measuring ratios of branching ratios • Need model for the shape of distributions to determine efficiency • Extremely hard: measuring cross-sections or branching ratios separately • Must generally do accounting for 100 percent of the produced particle’s decays But determining theory often requires cross-sections, branching ratios

14. What’s easy, what’s hard • Easy: • Measuring masses of resonances in e, mu, gamma • Measuring certain combinations of mass in dilepton decays • Measuring charge quantum numbers of particles • Hard: • Measuring masses of resonances in jets, taus • Measuring non-resonant masses directly – but see new methods! • Measuring spins of particles – but see new methods! • Extremely hard: • Measuring small mass differences • Measuring quark flavor quantum numbers (except t, b, maybe c) • Measuring mixing angles [requires many measurements] Unfortunately, extracting theory often requires masses and mixing angles

15. Example: Z’ resonance A 1.5 TeV electron-positron resonance could be discovered by December 2008, or June 2009 • Is it spin one? • What are its couplings to Quarks vs Leptons? • Special couplings to third generation? Tops? Bottoms? Taus? • Couplings to Right-handed vs Left-handed Fermions? • Does it decay to W+W-? • Does it decay to Z Higgs? • Does it decay to superpartners or other new particles? • Does it decay invisibly, and if so, can we determine what? So even if discovered right away, making a theory for Z’ will take years… • Lesson: expect to do model-building armed with fragmentary information

16. Everywhere at LHC: Jets, Jets, Jets Not all LHC events make *hard* (pT > 100 GeV) jets Still the probability of a pT ~ 20 GeV jet is very high • But what *are* jets? Naïvely: • quarks, antiquarks, gluons produced in scattering turn into jets because of confinement, hadronization D0 Dijet Event

17. Everywhere at LHC: Jets, Jets, Jets Not all LHC events make *hard* (pT > 100 GeV) jets Still the probability of a pT ~ 20 GeV jet is very high • But what *are* jets? Naïvely: • quarks, antiquarks, gluons produced in scattering turn into jets because of confinement, hadronization WRONG D0 Dijet Event

18. e+e-  quark-antiquark quarks q Z q

19. e+e-  quark-antiquark gluons q Z q

20. Lack of particle states in an interacting QFT • Once produced, a quark or gluon immediately begins to radiate • Nearly massless quarks • Spin-one radiation patterns • The radiation is dominantly collinear (along direction of motion) • Or soft (low energy, and subject to destructive interference) • Approximate conformal invariance • The process is scale invariant and forms a fractal pattern • Weak (but nonzero!) ‘t Hooft coupling (asNc) • The angular width of the fractal is small if the ‘t Hooft coupling is small Jet, pre-confinement: a narrow fractal distribution of (mostly) gluons • a fundamental object in an interacting gauge theory • Note this requires resummed perturbation theory

21. e+e-  quark-antiquark gluons q Z q

22. e+e-  quark-antiquark flux confined q Z q

23. Jets: role of confinement • Obviously, confinement has a role: • turn the fractal pattern of gluons into a jet of mostly mesons, a few baryons • But in fact • confinement in QCD has very little effect and • that this is critical for the phenomenon of jets How are these statements consistent? • To understand this, consider string theory: • Suppose I set an open string in motion in a particular state • In what circumstances might you directly observe the state at infinity?

25. If • open string coupling go = 0, • closed string coupling gc << 1, then typically string will • oscillate, • twist off closed strings – “gravitational radiation” • Initial state scrambled

26. If • open string coupling go << 1, • closed string coupling gc = go2, then typically string will • oscillate, • snap into few open strings • Initial state scrambled

27. If • open string coupling goND ~ 1, • closed string coupling gc = go2, then typically string will • Instantly breaks into many pieces • Initial state preserved

28. e+e-  quark-antiquark gluons q Z q

29. e+e-  quark-antiquark flux confined q Z q

30. e+e-  quark-antiquark hadrons q Z q

31. Strings vs. QCD Flux Tubes • Closed string coupling: gc vs. 1 / N2 • Open string coupling: go vs. 1 / N • Effective open string coupling: gcND vs. F / N • QCD has F = N= 3 • If F = 0, no jets! • If 0 < F << N, no jets? Maybe not… • Or quasi-jets, but jet momentum not ~ quark/gluon momentum Light flavors!

32. Summary of Jets and Confinement • N >> 1 and F << N good for non-perturbative aspects of QCD • But the failure of these same conditions allows parton-hadron duality which allows us to precisely test • Short-distance QCD scattering, decays of heavy particles (e.g. top), etc. • The semi-perturbative process of jet formation In short: We should not take the hadronic jets of QCD for granted!! • Too few flavors or many colors, confinement ruins jets • Too many flavors, no confinement and no hadrons • This is one of the reasons why jets are so ill-defined theoretically • [which means there’s more work to do!]

33. Standard Model Backgrounds • Almost every new physics signal has • a large standard model background or • a large detector background • or both • Experimentalists spend much of their time • Measuring backgrounds in data • Predicting backgrounds in advance of an analysis • Checking backgrounds in course of an analysis • A lot of theoretical calculation and simulation goes into this effort • Backgrounds are huge – • though fortunately they are smaller at high energy

34. Quick Review: Why do backgrounds fall? Backgrounds fall with energy

35. Quick Review: Why do backgrounds fall? Backgrounds fall with energy • Cross-section formulas - example: • All parton distribution functions fall like a power of x • Parton-parton c.m. energy ~ (x1 x2)1/2 (14 TeV) • Most parton-parton cross sections ~ 1/Energy^2

36. Quick Review: Why do backgrounds fall? Backgrounds fall with energy • Cross-section formulas - example: • All parton distribution functions fall like a power of x • Parton-parton c.m. energy ~ (x1 x2)1/2 (14 TeV) • Most parton-parton cross sections ~ 1/Energy^2 What’s this? The debris from the proton-proton collision! Unavoidably produced, always there. The “Underlying Event”!

37. CMS experiment: Simulated g g  Higgs  Z Z  e+e-m+m-+ underlying event!

38. CMS experiment: Simulated g g  Higgs  Z Z  e+e-m+m-+ underlying event!

39. Can QCD theory of proton structure predict properties of underlying event?!?!?! A challenge to formal theorists! Since we cannot currently model it, must measure it! One of first measurements this year (at 10 TeV) and next year (at 14 TeV): the properties of the average underlying event: how many particles? What pT distribution? Fluctuations in the underlying event are hard to measure – and can mask new physics All LHC predictions are affected by the underlying event; if underlying events are more accurate than is guessed, it would cause some problems for the experiments (Also every interesting proton-proton collision will be muddied by 4 – 20 simultaneous and boring ones ) CMS experiment: Simulated g g  Higgs  Z Z  e+e-m+m-+ underlying event!

40. What should theorists calculate? Tree-calculation solved – faster automation current goal But trees are always ambiguous, so need first quantum correction Dominant backgrounds are QCD multi-jet events, • so most important calculation a theorist can do is pure QCD…? No! • For fixed # jets, many processes contribute • # jets often does not equal # external legs • Multijet events are poorly measured Most measurements require at least one lepton or photon – they are “easy” • So jets + lepton (i.e., W or Z) or jets + photon are most important • State of art: W + 3 jets [4 in reach?] • But note: top-pairs = W + 4 jets already • Lots of signals are lepton + 4 jets [SUSY!] • Lots of signals are leptons + 6 jets [SUSY!] So there’s a long way to go – HELP!

41. What (not) to Compute • Calculating total cross-sections is easier for theorists • But measuring total cross-sections is all but impossible • Therefore theorists must provide differential cross-sections to allow these effects to be properly modeled • Harder for theorists; Analytic answers rare • Need to produce a computer program which can compute value of differential cross-section for a particular final state • Otherwise, experimentalists can only adjust the normalization of the tree-level calculation; shape still tree-level • Hope ds ~ dstree * (sloop/ stree) This can fail badly when looking at tails of distributions…

42. Unfortunately theory still has a long way to go here Aside from the fact that calculations are hard, there are deep conceptual problems (not new though, so not easy) • Fundamental problems of perturbation theory: • an asymptotic series in a running coupling – essential ambiguities • Radiation of multiple gluons; • breakdown of fixed-order perturbation theory • resummations in branching processes in initial, final state • Inability to quantify theoretical errors on any given calculation • Example: g g  Higgs boson (150 GeV) • LO: 15 pb • NLO: 25 pb • NNLO: 30 pb • There are important, challenging, understudied formal problems in quantum field theory here; they deserve more attention!

43. Finally -- the Signals of New Physics! ???!!!??? Let’s talk a little about Supersymmetry (SUSY) • a possible solution to the hierarchy problem, yes… • a favorite of string theorists • popular even 25 years ago, so the detectors were optimized to find it • (along with the Higgs and “technicolor”) • Instructive LHC lessons even if SUSY isn’t found at the TeV scale Now we have all heard SUSY  Missing Energy • i.e. Missing Transverse Momentum! and let’s recall why it is true […!...]

44. SUSY  Missing Energy+Jets+Leptons True in the simplest of the minimal SUSY models where • SUSY particles are odd under a new Z2 symmetry (“R-parity”) •  always produced in pairs •  decays of SUSY particles always have SUSY particles in final state •  the lightest one (“LSP”) can’t decay • The lightest one is neutral, colorless, and lives forever • LHC makes colored objects easily, colorless objects not • Therefore LHC makes gluinos and squarks most often • if they are not too heavy. • Decaying colored objects must dump color into the final state • But the LSP is neutral • So the color must exit as quarks or gluons •  JETS! Typically high pT • Often a partner of a Z or W is produced • This often allows for a lepton or two to be produced as well

45. SUSY  Missing Energy+Jets+Leptons Many Models have similar signatures. …or at least produce MET, high-pT jets and leptons in different combinations. True in the simplest of the minimal SUSY models where • SUSY particles are odd under a new Z2 symmetry (“R-parity”) •  always produced in pairs •  decays of SUSY particles always have SUSY particles in final state •  the lightest one (“LSP”) can’t decay • The lightest one is neutral, colorless, and lives forever • LHC makes colored objects easily, colorless objects not • Therefore LHC makes gluinos and squarks most often • if they are not too heavy. • Decaying colored objects must dump color into the final state • But the LSP is neutral • So the color must exit as quarks or gluons •  JETS! Typically high pT • Often a partner of a Z or W is produced • This often allows for a lepton or two to be produced as well

46. ATLAS detector: Supersymmetric event with jets, muons and MET – and U.E.

47. Missing Energy Especially Vague Almost a useless discovery by itself… and not even clear cut… • If event has missing transverse momentum, only can conclude • Something visible was mismeasured, or • Something visible went into a crack or near the beam, or • Something invisible was created and not observed But maybe just neutrinos? • If it’s a new neutral particle, that’s great! But hardly SUSY. • We don’t yet know if • The particle is a fermion or boson • The particle is produced in pairs • The particle is stable; lifetime > 10-8 sec, or decays to neutrinos? Expect long time from first claim of SUSY to convincing evidence!!

48. Is the MSSM Well-Motivated… ? Minimal SUSY [“MSSM”]: • Superpartners for all known particles • Two Higgs doublets, not one. • Supersymmetry is well motivated • Stabilizes mW/mPl hierarchy against radiative corrections • …As long as mu problem is solved… • Note – size of hierarchy NOT predicted • Minimality is not well motivated • Solves nothing • Makes theorists feel good – simplicity, beauty, elegance, Occham’s razor • But remember muon, 3rd generation, Z boson… • One should not give these two words equal weight!

49. Dangers of Minimalism For a theorist, adding one or two new particles may • Leave the main terms in the Lagrangian alone • Leave the key mechanism unchanged • Make the model look uglier • Make the model less predictive (more parameters) So theorists always like minimal models (easier to publish!) So do experimentalists (… why? …) For an experimentalist, adding one or two new particles may • Change the observable signatures 100 percent • Contradict the “lore” as to how to discover • Pose enormous challenges unrecognized in minimal model

50. Modifying the Higgs Sector A light Higgs boson is a very sensitive creature • New particles in loops can dramatically alter cross-sections, photon branching fraction • More scalars can generate mixing of eigenstates, new decay channels, new production mechanisms. Consider adding a single real scalarS to the standard model • S carries no charges and couples to nothing except the Higgs, through the potential