Signals And Backgrounds for the LHC

1. Signals And Backgrounds for the LHC -or- What They Were Thinking When They Designed ATLAS and CMS Thomas J. LeCompte Argonne National Laboratory

2. Lecture Series Outline • Original outline • A description of the ATLAS and CMS detectors • A laundry list of potentially interesting signals, followed by their backgrounds, stretching over two hours • This bored me to tears • I’d hate to think what it would do to you • Revised outline • I’ll try and explain why the detectors look like they do, in the context of “simple” physics measurements • I’ll discuss some early and interesting measurements Zzzzzz Think of this as one long talk, split over two days. STOP ME if I go too fast or you have questions!!

3. Outline • Four facts about detectors • Representative signals and how they influence experimental design • High pT muons • Higgs via H →gg • Jets • Top Quarks • High pT electrons • Missing ET and Exotica • Some early physics &future directions If you come away from this with the perspective that one experiment is better than the other, I haven’t done my job. I hope to outline what choices were made and why – not which choices were “right” I’m not kidding… STOP ME if I go too fast or you have questions!!

4. The Most Important Slide I Will Show jets From Claudio Campagnari/CMS Measured cross-sections (except for Higgs) at the Tevatron How to extrapolate to the LHC

5. “Nobody won any money betting against Michael Jordan” • I’m going to assume here that the Tevatron finds no new physics • Personally, I wouldn’t take this bet • The Tevatron is running very well • The experiments are experienced, and also running very well • Nonetheless, one has to assume something • If the Tevatron does find something, this talk becomes very simple: • Take whatever the Tevatron found • Make a zillion of them • Study the heck out of it

6. Fact One: The basic design of experiments is the same: Tracking • Driven by the physics of the interaction of high energy particles with matter. • Because the physics is the same, successful experimental designs are similar Calorimeters Muon detectors

7. The Compact Muon Solenoid

8. What CMS Looks Like Today

9. ATLAS = A Toroidal LHC ApparatuS

10. What ATLAS Looks Like Today

11. Fact Two: Tracking measures 1/p Charged particles in a uniform magnetic field move in helices: It’s convenient to work in the transverse plane (i.e. the plane normal to the Z direction)In this plane, the helices project to circles.

12. Fact Two: Tracking measures 1/p (II) The sagitta (“arrow”) s is the distance of maximum deflection from a straight line track: Radial line from origin to point where the particle exits the tracker: or Which leads to the expression As momentum increases, tracking becomes more difficult.

13. Absorber layers Sensitive layers Fact Three: Sampling Calorimeters • A (constant) fraction of the incoming particle’s energy gets converted to something that we can count (photons, electrons, etc…) • In the approximation that each layer absorbs a constant fraction of the energy, the calorimeter depth grows logarithmically with energy. As energy increases, calorimetry resolution improves.

14. Fact Three: Sampling Calorimeters (II) • EM showers all look the same • Hadronic showers are like snowflakes • Every one is unique A schematic of an electromagnetic shower A GEANT simulation of an electromagnetic shower

15. Fact Four: Compromise is a fact of life • Like it or not, experiments are constrained by resources • Every dollar that goes into one subsystem is a dollarthat doesn’t go into some other subsystem • Every inch that goes into one subsystem is an inchthat doesn’t go into some other subsystem • A collaboration with N members can’t design an experiment that takes 2N members to build or operate. • Industrial production capacity is finite • Evidence: crystals, silicon wafers, liquid noble gasses • Most experimenters have their own ideas on the best optimization • Individual interests and experience varies • The goal of a collaboration is to design a detector that everyone can live with – even if no single person thinks it’s ideal.

16. The Large Hadron Collider • The LHC: • Collides protons on protons • Crossing time of 25 (75) ns • Design Luminosity 1033 cm-2/s, increasing to 1034 after 1-2 years, and hopefully to 1035 (Super LHC) after that • The Tevatron • Collides protons on antiprotons • Crossing time of 396 ns • Design Luminosity 2 x 1032 cm-2/s – exceeded routinely.

17. Why Is The LHC More Luminous? • It has to be • No valence antiquarks in the proton. If you need an antiquark, you need to get it from the sea, or a gluon induced higher order process • It can be • It uses protons instead of antiprotons • Protons cost $3/oz. • Antiprotons cost $500,000,000,000,000,000/oz. • Energy is 7x higher • More beam bunches • LHC’s bunch intensity is very conservative, compared to what the Tevatron achieves routinely

18. LHC Stored Energy in Perspective LuminosityEquation: • Luminosity goes as the square of the stored energy. • LHC stored energy at design ~700 MJ • Power if that energy is deposited in a single orbit: ~10 TW (world energy production is ~13 TW) • Battleship gun kinetic energy ~300 MJ • It’s best to increase the luminosity with care USS New Jersey (BB-62) 16”/50 guns firing

19. Muon Detection:The Iron Ball

20. What Is The Iron Ball? • Suppose you wanted a detector to look for a very heavy (800 GeV) Higgs via the decay H →ZZ followed by Z → mm (both Z’s) • This is a very rare process → increase the luminosity • Handle this increased luminosity by only looking at muons: B-field region

21. CMS Muon Detectors • CMS uses the return field of their central solenoid to measure muon momenta • Four planes of detector stations inside the steel measure the muon’s tracks. • Low pT muons range out in the steel, providing an additional measurement.

22. The ATLAS Muon Spectrometer How muon trajectories bend in the magnetic field of the toroids. Energy stored in the magnetic field is ~1.2 GJ. Energy stored in a lightning bolt is ~1.5 GJ. Beam’s eye view Pictures from Jim Shank, Boston University

23. Comparing Design Philosophies Emphasizes • CMS uses as their magnetic field the return field through the iron • Allows one to go to large fields… • …which means small radii… (Figure of merit is BL2) • …which means that their calorimeter can be small (and expensive per unit volume). • ATLAS uses air core toroids • Very good resolution at high momentum • Up to ~TeV scale muons • Requires a lot of space B L2 Neither experiment is an iron ball, but elements of the iron ball design influenced both detectors.

24. Anticipated Muon Backgrounds • The main background to muons is other muons • What I mean is the main background to muons are muons from a source we are not particularly interested in. • Example: pion or kaon decays • Improving purity beyond ATLAS or CMS hardly helps. ATLAS

25. Unanticipated Muon Backgrounds • There is no such thing as a muon detector: • They are charged particle detectors, behind lots of steel • If there is any path around the steel, particles will find it • Since there are a million hadrons per muon, even if this is unlikely, it can be an important background • At the LHC, the cavern backgrounds (secondary particles) might be large. • The experiments have to worry about particles “raining down” into their detectors. • This is VERY difficult to predict from first principles. It has to be measured. The “CMX Ricochet”

26. Dimuon Mass ATLAS Requiring two muons is enough by itself to give a clean Z signal. The background is real muons – just from other sources. Note that this is a logarithmic plot

27. Dimuon Mass – Post Cuts It’s possible to remove virtually all of the background in the previous slide.(Exactly how is not really the subject of this talk)

28. Photon Detection

29. Higgs decays to diphotons From ATLAS Physics TDR • The background under the peak are real gg events: just not from Higgs decay • This is the irreducible background • Note the suppressed zero • To see the peak, there are two things you need to do: • Get the mass resolution as good as you can • make the peak narrow • Get the reducible background as small as you can • Keep the background from getting any worse than it already is

30. Higgs Event Displays ATLAS H → gg event CMS H → gg event

31. Why Pointing is Helpful (Part 1) g q Beam axis Can improve resolution g

32. Why Pointing is Helpful (Part 2) g q Beam axis Can reduce background g

33. Identifying Photons – Basics of Calorimeter Design Not too much or too little energy here. You want exactly one photon – not 0 (a likely hadron) or 2 (likely p0) Not too wide here. One photon and not two nearby ones (again, a likely p0) Not too much energy here. Indicative of a hadronic shower: probably a neutron or KL. A schematic of an electromagnetic shower A GEANT simulationof an electromagnetic shower

34. ATLAS Electromagnetic Calorimeter Design resolution: Technology: uses lead as an absorber and liquid argon as an ionization medium. Energy deposited in the calorimeter is converted to an electrical signal.

35. ATLAS Liquid Argon Calorimeter Module • Highly segmented • Allows measurement of shower development • Rejects background • Has some pointing ability • Very good (but not as good as CMS) energy resolution • “Accordion” faster than other LAr calorimeters • Still slower than crystals

36. ATLAS Calorimeter in Real Life Before installation – it’s now in a cryostat and impossible to see.

37. CMS Calorimeter Crystals 16X0 • CMS uses Lead Tungstate crystals • Scintillator: energy is converted to light • Exceptional energy resolution, because there are no inert absorbers • The focus is to get the best possible energy resolution, no matter what it takes • Energy resolution is ~2x better than ATLAS’ in the region where Higgs decay is important 22X0 26X0 Design resolution: Another nice feature – low noise Photo: Ren-yuan Zhu, Caltech

38. CMS EM Calorimeter Figure: Ren-yuan Zhu, Caltech

39. Comparing Design Philosophies • CMS emphasizes energy resolution • Use PWO crystals • Expensive – means go to small radius to keep the detector within budget • Only handful of vendors worldwide • ATLAS emphasizes background rejection • Able to go to larger radius: separates showers better • Highly segmented calorimeter allows measurement of shower development • One photon? Two? A hadron masquerading as a photon? • Both calorimeters are quite thick • Improves resolution (showers are contained) • Degrades electron-hadron separation • ATLAS measurement of shower development is intended to compensate

40. Jets When you’re a jet, you’re a jet all the way… S. Sondheim

41. A Two Slide Review of Jets A “blast” of particles, all going in roughly the same direction. 2 jets 2 jets 2 2 5 3 3 jets 5 jets Same Events, Tracking View Calorimeter View

42. More on Jets • Where do they come from? • The force between two colored objects (e.g. quarks) is independent of distance • Therefore the potential energy grows linearly with distance • When it gets big enough, it pops a quark-antiquark pair out of the vacuum • These quarks and antiquarks ultimately end up as a collection of hadrons • Process is called “fragmentation” or “hadronization” g g g g One (of several) processes that produce jets in collisions.

43. Measuring Jets • Lego plot of a top quark event has EM calorimeter energy in green and hadronic calorimeter energy in red. • The prevalence of green comes from two facts: • Half of the particles (and ~40% of the energy) in a jet are photons from neutral meson decay • The LHC EM calorimeters are thick, and many hadrons begin their showers inside the electromagnetic calorimeters. ATLAS

44. ATLAS “Tile” Hadronic Calorimeter Uses steel as an absorber, and scintillating tiles as the active medium. Energy is converted to light.

45. CMS HCAL (Hadronic Calorimeter) • Also a scintillating tile-based sampling calorimeter • Technology is similar to ATLAS: • Absorber is brass instead of steel • Tile orientation is different (more conventional in CMS) • Calorimeter is relatively thin

46. Comparing Design Philosophies • CMS’ calorimeter is inside their magnet • Additional depth is very expensive (since it requires a larger radius magnet) • To increase the effective thickness, it’s made of a denser material (brass) • ATLAS is quite thick – the outer muon detector resembles the “iron ball” design. • ATLAS would “naturally” use liquid argon for both the hadronic and electromagnetic calorimeter • Both D0 and H1 designed their detectors this was • This is also very expensive • Both experiments have chosen to economize here • Slightly better performance would cost a lot more money

47. Jets & Hadron Calorimetry Many things besides hadronic calorimeter response affect the jet energy determination: the EM calorimeter response, out of cone corrections, dead material and dead material corrections, hadronic decays or interactions upstream of the calorimeter… Event with the best calorimeters, you are taking a very clear picture of a very fuzzy object. Economizing on the hadron calorimeter is often the “least bad” option.

48. Tops

49. Top Quark Pair Production Top pair events are characterized by the decay of the two W’s in the event.

50. Early Top Quarks at the LHC • Start with a lepton (e or m) plus four jets sample. • Make all three jet mass combinations, requiring m(jj) = m(W) • Identifying one jet as containing a b quark (“b-tagging”) is not required. Why is the top signal so clear without b-tagging? Especially since the Tevatron needed b-tagging to discover the top quark?