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Road to Discovery: Lecture 1

Please note that this road approaches its destination slowly, for safety reasons. Road to Discovery: Lecture 1. Sarah Eno U. Maryland. Credit where credit is due. I stole most of my slides from

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Road to Discovery: Lecture 1

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  1. Please note that this road approaches its destination slowly, for safety reasons. Road to Discovery: Lecture 1 Sarah Eno U. Maryland CERN-FNAL HCP Summer School

  2. Credit where credit is due I stole most of my slides from • Chris Hill,BeateHeinemann,JoeIncandela,RobertoTenchini, Andy Parker, TommasoLari, SilvanoTosi, Klaus Rabbertz, Kostas Kousouris, Joey Huston, J. Fernandez, Sasha Nikitenko, SaeidPaktinat, PiotrZalewski, Barbara Clerbaux, Christian Bauer, Roberto Rossin, AlexeyDrozdetskiy, MojatabaNajafabadi, Peter Richardson, FedorRatnikov, Valerie Halyo, Seong Chan Park (and people I’ve forgotten to mention) • Also, note specific citations of articles CERN-FNAL HCP Summer School

  3. Outline • Lecture 1: Lessons from an old person, Standard Model at LHC • Lecture 2: Early Exotica • Lecture 3: SUSY & Higgs CERN-FNAL HCP Summer School

  4. CMS/D0 Bias I am currently a member of both the D0 and CMS collaborations, so its just easiest for me to get examples. Most of the results are similar between D0/CDF and CMS/ATLAS. I’ll try to note where major differences are expected. (CDF and AMY in my younger days) CERN-FNAL HCP Summer School

  5. Discover what? An exciting time, as there are so very many questions: • Questions of the standard model • What causes electroweak symmetry breaking? • Where is the higgs? • Why is it light? • Why is there so much more matter than antimatter in the universe?? • Question from cosmology • Where is the dark matter particle? • What is dark energy? • And Beyond • Why do the fermions have such different masses? • Why are neutrinos so light? • Why are neutrinos not masses? • Do the forces unify at a high energy scale? • etc. etc. etc. CERN-FNAL HCP Summer School

  6. Discovery at the LHC* • Simple, right? LO description of a new particle search • think about what distinguishes your signal from background • design some variable or combination of variables (boosted decision tree, NN, etc) that maximizes the differences • cut away most of your background using some criteria that optimizes significance • estimate the remaining number of SM events (production cross section, BR, acceptance, efficiencies) • see how many pass and feed that and your estimated background to RooStat and see what you got • But of course the devil is in the details. • need to use well-understood variables in a well-understood detector for well-understood background processes Look at past devils. Look at best guess of our own devils. (* will concentrate on “discovery” through new particle searches in these lectures. Obviously, much important new physics can be done in B physics, QCD, etc as well.) CERN-FNAL HCP Summer School

  7. It’s been a long time… 1983: Tevatron reaches 500 GeV 1985: Tevatron reaches 800 GeV 1987: First collisions in CDF 20 years! 1982 1987 Our lessons come from long ago. Changing times will affect these lessons. Installing main ring dipole magnets Colloqium, Boston University

  8. History of Discoveries Even though we used to cross the energy frontier regularly, major discoveries come about once/decade. c,τ b CERN-FNAL HCP Summer School

  9. CERN Z0 1983 Top and W/Z Tevatron, top, 1995 Colloqium, Boston University

  10. sps Z0 Very Different? The SppS turned on at 1% of final instantaneous luminosity in 1983, but in the first run of a few months discovered the W with 8 events. Tevatron turned on in 1987. Top discovered 8 years later. joint MD-Hopkins Mtg

  11. Actually had a lot in common • ALL properties of the signal tightly constrained by standard model • Signal shows up in multiple channels • with FIXED RELATIONSHIPS • not only does the background agree with the background prediction, but THE SIGNAL AGREES WITH THE SIGNAL PREDICTION • searches are easy (in principal) to design (optimize s/root(b), and it’s (relatively) easy to interpret the results joint MD-Hopkins Mtg

  12. Z to bb When what you are looking for has very well defined properties, you can do remarkable things Would you believe this if it weren’t the Z? joint MD-Hopkins Mtg

  13. Single top CDF CERN-FNAL HCP Summer School

  14. Tau/charm discovery When the new physics is unexpected, things are much more challenging http://www.osti.gov/accomplishments/perl.html CERN-FNAL HCP Summer School

  15. tau/charm CERN-FNAL HCP Summer School

  16. “Discoveries” There are good reasons to be sceptical about new discoveries! When looking for something that is not well predicted, it is much easier to make mistakes (missed systematic, cut tuning) or be mislead by a statistical fluctuation. • That went away • contact interactions • pentaquarks • leptoquarks • 40 GeV top quark • zeta • Mark J events • eeγγ event That were missed: Many interesting, apocryphal stories involving overflow bins, a frantic sorting of stacks of punch cards … stories people only discuss after much beer. (Could CLEO could have caught this before Babar?) CERN-FNAL HCP Summer School

  17. Discrepancy? When we see disagreement, what can it be? * theory cross section for SM processes wrong? * theory kinematic distributions for SM processes wrong? * particle ID fake rate is wrong? * particle ID efficiency is wrong? * energy scales are wrong? * resolution is wrong? When you have eliminated the impossible, whatever remains, however improbable, must be the truth (Sherlock Holmes) CERN-FNAL HCP Summer School

  18. How to minimize devils • How can we minimize these errors: • ratios of cross sections • get efficiencies and fake rates energy scales, resolutions from data whenever possible • have a strong group tuning the MC (both theory of background production properties and detector resolution in close contact so as to not mistake one for the other.) • have many data-based methods for checking the background calculation • Most important advice to young particle hunters based on past experiments? • know your detector • know your backgrounds and ALL the sources of their uncertainties CERN-FNAL HCP Summer School

  19. (Nice comparison by Chris Hill) CERN-FNAL HCP Summer School

  20. CERN-FNAL HCP Summer School

  21. No detector ever had this much material before the calorimeter. D0 had 0.3 X0 CERN-FNAL HCP Summer School

  22. CERN-FNAL HCP Summer School

  23. CERN-FNAL HCP Summer School

  24. Before Beam: Cosmics ATLAS and CMS may be the best understood detectors ever at turn on. "Shedding Light on Dark Matter", U. MD.

  25. Beam Splash Events • Beam with 2x109 protons dumped onto a target (collimator)150m upstream of CMS • Sept. 7,9,10,18 • Leads to a “tsunami” wave of O(100K) muons coming down the tunnel! • A far cry from the single cosmic muon events… TCTH TCTV TCLP TAN MBRC CMS 146 m "Shedding Light on Dark Matter", U. MD.

  26. CMS lights up HCAL energy ECAL energy debris DT muon chamber hits Inner tracking systems kept OFF "Shedding Light on Dark Matter", U. MD.

  27. ATLAS also lights up "Shedding Light on Dark Matter", U. MD.

  28. Well-calibrated CERN-FNAL HCP Summer School

  29. Well-Commissioned Detectors Atlas and CMS detectors commissioning data: better tuned than any large detector! (at least that I've worked on) however, even well-tuned detectors show surprises D0 Run II Z mass and MET (every body likes to show this, however D0 Run II was not a well-tuned detector) But probably more interesting is D0 Run I Z mass (this was also a well-tuned detector) CERN-FNAL HCP Summer School

  30. Can still do a lot with a new detector, even with surprises Run Aug – Oct 92. Some plots from a Nov 92 talk (Madaras) Peak at 86 GeV. Don’t panic. Rescale the EM energy scale and move on. CERN-FNAL HCP Summer School

  31. Can still do a lot with a new detector Run Aug – Oct 92. Some plots from a Nov 92 talk (Madaras) (MET obviously was quite useable in this brand new detector.) W events CERN-FNAL HCP Summer School

  32. Can do a lot with new detectors • In 1983, UA1/UA2 discovered theWat 1% of design luminosity within a few months of running. • First publications by UA2 were “Production and Properties of Jets” (1982) and “Discovery of the W” (1983) • The Tevatron (with CDF only) had a small run in 1987 Run, (33 nb-1). The first search presented by CDF was a publication submitted 24 July, 1989 (http://prola.aps.org/abstract/PRL/v63/i14/p1447_1) of a search for heavy stable particles, done with 26.2 nb-1 of data. • The Tevatron (with CDF only) had another run the next year ( June 88 - June 89). The first search was a paper on the search for the top quark, submitted 13 Oct 1989 (see http://prola.aps.org/abstract/PRL/v64/i2/p142_1) • D0 first took data during the Tevatron's "Run I". 1.1 pb-1 of data was taken from August to October 1992 and was presented in November in conferences (see, for example, http://lss.fnal.gov/archive/1992/conf/Conf-92-365.pdf) (albeit with D0 showing a Z peak at 86 GeV!). Results on new particle searches (top quark search) were presented at conferences even after this early running. This run eventually accumulated 200 pb-1 of data. (Note two detectors were running at this time). The first paper was a leptoquark search.

  33. Devils Enough generalities… What will be the challenges specific to the LHC? CERN-FNAL HCP Summer School

  34. Lots of Backgrounds • Most of the backgrounds can be made small by requiring an isolated e or mu, or large MET, in the final state. (apologies for neglecting taus in all these talks) • (However, there can be large uncertainties on the remaining “QCD” background. In general, the simulation can not reliably predict these backgrounds because • fake rates from MC are generally good to within a factor 2 at best • tails of MET distribution even less reliable • need higher order QCD calculations for kinematics, which are not (all) available • due to the small fake rate, would need huge MC samples to do this • Data-based background methods are required.) CERN-FNAL HCP Summer School

  35. Cross sections After lepton or large MET requirement, main backgrounds tend to be top, W, Z, dibosons Also see arXiv:hep-ph/0611148 CERN-FNAL HCP Summer School

  36. Machine Plan 50-300 pb-1 at 9-10 TeV http://lhc-commissioning.web.cern.ch/lhc-commissioning/luminosity/09-10-lumi-estimate.htm CERN-FNAL HCP Summer School

  37. 10 vs 14 TeV Unfortunately, most of the studies done so far are at 14 TeV. ratio of parton luminosities plots gg of course down more than qq Stirling CERN-FNAL HCP Summer School

  38. 10 TeVvs 2 TeV How much luminosity do we need to beat 8 fb-1 (now 5) at the tevatron? The tevatron can probe up to sqrt(s) = 2 TeV, so its not a spent machine. 26 O(5) events for discovery O(8) before acceptance O(30) fbxsec at these masses Stirling CERN-FNAL HCP Summer School

  39. Devil 1: Measuring Luminosity • How many background events do we expect? Need to know luminosity • relative luminosity by rate of inelastic collisions (measured in some sort of forward detector) • absolute normalization? CERN-FNAL HCP Summer School

  40. Luminosity Measurement 10 TeV TOTEM experiment will measure this directly, but its not a trivial measurement. CERN-FNAL HCP Summer School

  41. Dead Reckoning 10% CERN-FNAL HCP Summer School

  42. Underlying event • The first thing we will see is a few soft scatters (MB events). Generally, these are not backgrounds for our searches. However, these are useful for our search preparations, as they tell us about the UE. The UE: • affects efficacy of isolation variables • affects MET resolution, especially for smaller values of MET • may be quicker in the beginning to measure effects from data than try to retune the MC. CERN-FNAL HCP Summer School

  43. Minimum Bias Model expectations for charged particles at |η| =0 vs. √s: Pythia: ~ ln2(s)‏ Phojet: ~ ln(s)‏ CERN-FNAL HCP Summer School

  44. Properties of MB events CMS ATLAS Track reconstruction How will you handle the news from these analyses in your search? CERN-FNAL HCP Summer School

  45. Jets • Next we’ll see production of high pTdijets, multijets. • calibration and understanding of jets for searches • checks on PDF’s, ISR, FSR in new environment • searches for new particles decaying to dijets This is not my generation’s QCD. Much better calculations. But, it will pay to understand them and their limits. Jets will give us our first indication as to how well these calculations are doing at the new energy. CERN-FNAL HCP Summer School

  46. dijets The LHC will be a very jetty place. NLO code by NLOJET++, Z. Nagy Tevatron limit Inclusive kT cross section in 6 bins in rapidity y, D = 0.6 LHC @ 10 TeV 1 event in 10 pb-1 1 event in 300 pb-1 NLOJET++, PRL 88 122003 (2002)‏ PR D68 094002 (2003)‏ fastNLO, hep-ph/0609285 CERN-FNAL HCP Summer School

  47. Jet Cross Section A non-trivial measurement: both theory and experiment have non-negligible uncertainties. CERN-FNAL HCP Summer School

  48. Uncertainties CERN-FNAL HCP Summer School

  49. Theory Uncertainties CERN-FNAL HCP Summer School

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