Early LHC Physics: Theory

1. Joint Theory-CMS Seminar, Fermilab, May 31st 2007 Early LHC Physics: Theory Preparations for Peter Skands Fermilab / Particle Physics Division / Theoretical Physics

2. Overview • Introduction • A recipe for hadron collisions • Models by the number, tunes by the score, and none the wiser? • Theoretical implications of early LHC physics once … • I got my tracker working • I got my muon chamber working • I got my jets working • I got my photons working (+ electrons) • I got my mojo working • Who is who Event Generator Status

3. QuantumChromoDynamics • Main Tool • Matrix Elements in perturbative Quantum Field Theory • Example: CMS ATLAS Reality is more complicated Event Generator Status

4. A Recipe For Hadron Collisions Hadron Decays Non-perturbative hadronisation, colour reconnections, beam remnants, non-perturbative fragmentation functions, pion/proton ratio, kaon/pion ratio, Bose-Einstein correlations ... Soft Jets + Jet Structure Multiple collinear/soft emissions (initial and final state brems radiation), Underlying Event (multiple perturbative 22 interactions + … ?), semi-hard separate brems jets Exclusive & Widths Resonance Masses … Hard Jet Tail High-pT wide-angle jets Inclusive s • + “UNPHYSICAL” SCALES: • QF , QR : Factorisation(s) & Renormalisation(s) Event Generator Status

5. A recipe for hadron Collisions • Experimental Approach (naïve view) • Inclusive σpp(14 TeV) Trigger(s) + Calibrations  Analysis cuts • Measure distributions on the accepted events • Taking statistical fluctuations and the risk of systematic mismeasurement into account to the best of one’s ability  (Correlated) Uncertainty Estimate • Now you want to compare to ‘theory’ but what is that? • Prohibitive to calculate entire S matrix path integral directly! • Divide and conquer: calculate small parts of the S matrix at a time • Cross section = sum over “subprocesses” (w/o interference) • 1 subprocess = [sum at amplitude level]2 (i.e. w/ interference) • Signal • Other processes which are known to contribute “significantly” • “Unknown” unknowns: mis-ID, “reducible backgrounds”, detector effects, (BSM?), … • At each step, do further expansions, further approximations, … • Even the most sophisticated calculation can be disastrously wrong Event Generator Status

6. Models by the number, Tunes by the score … More Physics: Multiple interactions + impact-parameter dependence UA5 @ 540 GeV, single pp, charged multiplicity in minimum-bias events Sjöstrand & van Zijl PRD36(1987)2019 Simple physics models ~ Poisson Can ‘tune’ to get average right, but much too small fluctuations  inadequate physics model • Morale (will return to the models later): • It is not possible to ‘tune’ anything better than the underlying physics model allows • Failure of a physically motivated model usually points to more, interesting physics Event Generator Status

7. Overview • Introduction • A recipe for hadron collisions • Models by the number, tunes by the score, and none the wiser? • Theoretical implications of early LHC physics once … • I got my tracker working • I got my muon chamber working • I got my jets working • I got my photons working (+ electrons) • I got my mojo working • Who is who Event Generator Status

8. Disclaimer • I’m not an expert, so will not focus on: • CMS: triggers, calibrations, analyses, systematics, etc. • Theory: PDF constraints, BSM physics, discovery potential, etc. • All of these aspects are vital, and will be carried out by abler people • Instead, I will try to: • Give you a pick of some interesting early possible measurements • Take a look ‘under the hood’ of these and discuss their role, if any, in understanding the universe better (including but not restricted to better calibrations) • Note: this is not ancient material • It is fresh off the farm, reflects the uncertainties of current state-of-the-art (or at least the subset obtainable with Pythia) • All plots below are shown for the first time in this talk Event Generator Status

9. Counting Tracks • Theory “predictions” for tracker occupancy (idealized 4π tracker): • 106 events will immediately yield a lot of information • This was theory – how related to what is more realistically measured? • Restrict to |η| < 2.5, pT > 0.5 GeV <Nch> ~ 80-120 A bunch of models and tunes Event Generator Status

10. Counting Tracks • Theory “predictions” for tracker occupancy: • 106 events will still immediately yield a lot of information, but not all • Extrapolation to inclusive distribution quite model-dependent, interesting to extend measurements as far as they can possibly go (e.g. ATLAS talks about possibly ~ 150 MeV using Silicon, but this probably does not fall under “early” LHC physics …) <Nch> ~ 13-20 Event Generator Status

11. Under the hood (theory) The things that tracks track Spacetime Matter overlap vs. Nch(~ transverse density profile folded on itself) Average number of “colour sparks” per pp collision vs. Nch <Nint> ~ 4 - 11 Event Generator Status

12. Knowns and Unknowns • The Nch distributions are sensitive to • The spatial distribution of the proton mass • QCD cross sections folded with multiparton densities • Hadronization (+ string interactions? Other infrared physics?) • Energy / Collider extrapolation of all of above • A multi-dimensional theory space  better precision • Look at many variables  successively rule out models • Pedestal effect • Forward-Backward multiplicity correlations • Underlying Event, Rick’s “Transverse” minijets, … (active min-bias similar to UE) • LEP, HERA, …, but then additional uncertainties from extrapolation to different initial states Earliest on: pT and η distributions for charged particles Event Generator Status

13. Multiplicity vs pT correlation • Extremely interesting already at Tevatron and RHIC (even UA5) • An independent / incoherent set of strings  flat distribution •  first indication of interacting strings? • If so, is this a consequence of the area law for classical strings? Event Generator Status

14. Multiplicity vs pT correlation • Persists (but again changed) when restricting to |η| < 2.5, pT > 0.5 GeV • An independent / incoherent set of strings  flat distribution •  first indication of interacting strings? • If so, is this a consequence of the area law for classical strings? Event Generator Status

15. The (QCD) Landscape hadronization bbar from tbar decay pbar beam remnant p beam remnant qbar from W q from W q from W b from t decay ? Triplet Anti-Triplet Structure of a high-energy collision In reality, this all happens on top of each other (only possible exception: long-lived colour singlet) D. B. Leinweber, hep-lat/0004025 Event Generator Status

16. Overview • Introduction • A recipe for hadron collisions • Models by the number, tunes by the score, and none the wiser? • Theoretical implications of early LHC physics once … • I got my tracker working • I got my muon chamber working • I got my jets working • I got my photons working (+ electrons) • I got my mojo working • Who is who Event Generator Status

17. Drell-Yan • Muons are a direct light into the heart of the process • Very clean, highly controllable probe of Initial-State Radiation (ISR) • Feed back into photon + jet  improve jet calibration • Muons + tracking •  can also study Underlying Event here (minijets, fragmentation, …) • Evolution of UE and ISR as a function of Q2  good model constraints Event Generator Status

18. Drell-Yan • Muons are a direct light into the heart of the process • Very clean, highly controllable probe of Initial-State Radiation (ISR) • Feed back into photon + jet  improve jet calibration • Muons + tracking •  can also study Underlying Event here (minijets, fragmentation, …) • Evolution of UE and ISR as a function of Q2  good model constraints K. Lassila-Perini (Moriond 2006) hep-ex/0605042 Event Generator Status

19. Overview • Who is who • Introduction • A recipe for hadron collisions • Models by the number, tunes by the score, and none the wiser? • Theoretical implications of early LHC physics once … • I got my tracker working • I got my muon chamber working • I got my jets working • I got my photons working (+ electrons) • I got my mojo working Event Generator Status

20. Jet rates • Jet rates: 106 min-bias + 105 pThat > 20 GeV • First: no huge “hard” differences for first couple of jets. • Second: Shapes and ratios, e.g. N3/N4, do have information • Third: isolate special regions, group jets pairwise, j-j angles, etc … 2J20 MB Event Generator Status

21. Once everything is working, put it all together Missing ET, HT Dark Matter And all the other fun stuff It may be a (long) while till the next collider   Turn LHC into a precision machine Example: beautiful Tevatron measurements such as the W and top masses are already providing crucial information Got My Mojo Working Event Generator Status

22. The Road to Higher Precision • Go further into perturbative series • Matching + better-understood resummations • Very high precision on perturbative part can be expected in the long term •  less ‘wriggle room’ for non-perturbative part? • Beyond LEP fragmentation models • K-not-so-short? • Is there a strange light at the end of the tunnel? • Is it the LEP tunnel? • Lambdas: • How do baryons get made? • And did people at LEP make them the same way we make them? • Tracking at very low momenta (150 MeV?) • How much charged energy sits at low momenta? RHIC Anomaly STAR: neither strangeness nor baryon production appears to be well described RICK Anomaly Rick (Field): the charged/neutral ratio does not appear to be well described Event Generator Status

23. Conclusions • When you see the first LHC collisions, take a moment to have pity on the theorists who will have to wait • At 14 TeV, everything is interesting • When you see that first event with 300 charged tracks in it, pause a moment to try to visualize what it must have looked like up close • Early LHC Physics: theory • Even if not a dinner Chez Maxim, rediscovering the Standard Model is much more than bread and butter. • Real possibilities for real surprises • It is both essential, and I hope possible, to ensure timely discussions on “non-classified” data, such as this  allow rapid improvements in QCD modeling (beyond simple retunes) after startup Both these and Kevin’s slides will be availabe on the theory seminar web page theory.fnal.gov/seminars Event Generator Status

24. (pT) Event Generator Status

25. (eta) Event Generator Status

26. Who is Who In terms of current work (obviously somewhat sketchy, floating borders …) • B Physics, flavor physics, SCET, HQET, form factors, … • Thomas Becher, Andreas Kronfeld • Enrico Lunghi, Richard Hill, Jack Laiho, Ruth van de Water • Lattice QCD, chiral perturbation theory, hadron spectra, … • Bill Bardeen, Estia Eichten, Andreas Kronfeld, Paul MacKenzie, Chris Quigg, Jim Simone • Ruth van de Water, Jack Laiho, Elizabeth Freeland • Perturbative QCD, NLO, jet physics, event generators • R. Keith Ellis (MCFM), Walter Giele (VECBOS, VINCIA), Peter Skands (PYTHIA, VINCIA) • Steven Mrenna (CMS, PYTHIA, …) • Jan Winter • Leaving this fall • Arriving this fall • Often here on Thursdays Event Generator Status

27. Who is Who In terms of current work • SM Higgs & MSSM phenomenology • Bill Bardeen, Marcela Carena, Estia Eichten, Joe Lykken, Chris Quigg, Peter Skands* • Stephen Martin, Ayres Freitas*, Carlos Wagner • non-SM Higgs, non-MSSM BSM (Xtra Dim, Techni, Little H, …) • Bill Bardeen, Marcela Carena, Bogdan Dobrescu, Chris Hill, Joe Lykken, Chris Quigg, Estia Eichten, Tim Tait • Richard Hill, Jay Hubisz, K.C. Kong, Rakhi Mahbubani, Jose Santiago • Neutrino Physics • Carl Albright, Boris Kayser, Stephen Parke • Andre de Gouvea, • + Ties to astro / cosmo / dark matter • Leaving this fall • Arriving this fall • Often here on Thursdays Event Generator Status