1. HEP seminar, JHU, March 2007 Peter Skands Recent Developments in QCD Collider Phenomenology

2. Peter SkandsEvent Generator Status 2 D. B. Leinweber, hep-lat/0004025 The (QCD) Landscape Anti-Triplet Triplet pbar beam remnant p beam remnant bbar from tbar decay b from t decay qbar from W q from W hadronization ? q from W Structure of a high- energy collision In reality, this all happens on top of each other (only possible exception: long-lived colour singlet)

3. Peter SkandsEvent Generator Status 3 ► Introduction QCD at colliders & event generators ► Perturbative QCD Matching & VINCIA ► The Underlying Event Towards a complete model of hadron collisions Colour reconnections and precision measurements ► OutlookOverview

4. Peter SkandsEvent Generator Status 4 Q uantum C hromo D ynamics

5. Peter SkandsEvent Generator Status 5 Traditional Event Generators ►Basic aim: improve lowest order perturbation theory by including leading corrections  exclusive event samples 1.sequential resonance decays 2.parton showers 3.underlying event 4.hadronization 5.hadron and tau decays

6. Peter SkandsEvent Generator Status 6 The Event Generator Position

7. Peter SkandsEvent Generator Status 7 The Monte Carlo Method Hard Part Up to E cm Parton Showers + Multiple Interactions Multi-GeV Hadron Decays Hadronization + Remnants ~ 1 GeV ~ 10 -15 m σ hard process, P res P ISR, P FSR, P MI P remnants, P hadronization P decays

8. Peter SkandsEvent Generator Status 8 Words of Warning ►[…] The Monte Carlo simulation has become the major means of visualization of not only detector performance but also of physics phenomena. So far so good. But it often happens that the physics simulations provided by the Monte Carlo generators carry the authority of data itself. They look like data and feel like data, and if one is not careful they are accepted as if they were data. ►[…] They do allow one to look at, indeed visualize, the problems in new ways. But I also fear a kind of “terminal illness”, perhaps traceable to the influence of television at an early age. There the way one learns is simply to passively stare into a screen and wait for the truth to be delivered. A number of physicists nowadays seem to do just this. J. D. Bjorken from a talk given at the 75th anniversary celebration of the Max-Planck Institute of Physics, Munich, Germany, December 10th, 1992. As quoted in: Beam Line, Winter 1992, Vol. 22, No. 4

9. Peter SkandsEvent Generator Status 9 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 Resonance Masses … Hard Jet Tail High-p T wide-angle jets & Widths + “UNPHYSICAL” SCALES: Q F, Q R : Factorisation(s) & Renormalisation(s) s Inclusive Exclusive Hadron Decays Collider Energy Scales

10. Peter SkandsEvent Generator Status 10 T he B ottom L ine The S matrix is expressible as a series in g i, ln(Q 1 /Q 2 ), ln(x), m -1, f π -1, … To do precision physics: Identify, compute, and control all large terms Solve more of QCD over all of phase space: fixed order + resummations Control it answer should include reliable estimate of uncertainties and be systematically improvable Non-perturbative effects don’t care whether you know how to calculate them

11. Peter SkandsEvent Generator Status 11 Pre-LEP Event Generators ► “Seated on a tripod above a crack in the earth, she went into a trance by the stupefying vapors rising from the earth and by chewing laurel leaves. From the incoherent babbling which the priestess spoke in her ecstasy, the temple priests formulated the oracle.” ►There was a certain amount of voodoo involved ►Still, an impressive amount of data could be accounted for ►We now have much better voodoo The Pythia ►ARIADNE, HERWIG, and PYTHIA T. Sjöstrand, S. Mrenna, PS, JHEP 05 (2006) 026

12. Peter SkandsEvent Generator Status 12 Developments since LEP ►Hard Bremsstrahlung (additional jets, e.g. Z  3,4,5 jets) Parton shower approximation breaks down  large uncertainties Lots of progress in last ~ 5 years: “matching” to matrix elements ►Jet Broadening (soft/collinear parton bremsstrahlung) New generation of theoretical models (+ comparisons to analytic results) Improved parton showers implemented in both general-purpose generators (HERWIG(++) and PYTHIA) + work at Fermilab: VINCIA – a next-generation “antenna” shower ►The Final Femtometer – Hadronization With short-distance parts better under control  non-perturbative side has less wriggle room  also here improved possibility for studies Current studies at hadron colliders  something interesting ? X + jets Jet mass High Precision

13. Perturbative QCD Matching & VINCIA

14. Peter SkandsEvent Generator Status 14 ►Starting observation: forward singularity of bremsstrahlung spectra is process-independent. Fundamental property of gauge theory  also applies to gluon brems. Leading contributions to both QED (photon radiation) & QCD (quark, gluon radiation) can be worked out to all orders once and for all  exponentiated (Altarelli-Parisi) integration kernels ►Iterative (Markov chain) formulation = parton shower can be used to generate the forward singular (“collinear”) parts of QED and QCD corrections to any process to infinite order in the coupling ordered in a measure of resolution  a series of successive factorizations the lower end of which can be matched to a hadronization description at some fixed low hadronization scale ►Limitations misses interference terms relevant in the deep non-singular region kinematic ambiguities and double counting between fixed order part and resummed part Bremsstrahlung: Parton Showers σ n+1 = σ n P n  n+1 Forward (collinear) factorization

15. Peter SkandsEvent Generator Status 15 The Problem ►The best of both worlds? We want: A description which accurately predicts the rates of hard additional jets (e.g. W+1j, W+2j, W+3j, etc) And which simultaneously describes the jet structure and the effects of multiple soft emissions (e.g. p TZ, jet broadening) ►How would you do it? (example: W production) Parton shower approach: start from W production, generate additional jets by parton showering Problem: parton shower misses relevant terms for hard jets, rates only correct for strongly ordered emissions p T1 >> p T2 >> p T3... (common misconception that showers are soft, but that need not be the case. They can err on either side of the right answer.)

16. Peter SkandsEvent Generator Status 16 Example: tops, gluinos, and squarks plus jets T. Plehn, D. Rainwater, PS - hep-ph/0510144

17. Peter SkandsEvent Generator Status 17 More problems ►So a pure parton shower is not enough, But we know that: The missing terms can be calculated up to fixed orders in perturbation theory, from matrix elements (e.g. ALPGEN, CalcHEP, MadGraph)  calculate W+j matrix element, start a parton shower from it? This is correct for inclusive W+j observables But now it does not describe the W+0j bin - if we take p T of the first jet very small: collinear singularities in matrix element  infinities It also does not correctly describe the W+2j and higher bins (unless p T2 << p T1 ) ►So what we would really like is to combine many different matrix elements (for W, W+j, W+2j, etc) with parton showers in a consistent way

18. Peter SkandsEvent Generator Status 18 Double Counting ►For a fixed jet multiplicity, we know what to do ☺ ►So combine different multiplicites  inclusive sample? ►In practice – Combine 1.[W] ME + showering 2.[W + jet] ME + showering 3.… ►But this would give double counting: Above, we saw that [W] ME + showering produces some W + jet configurations (albeit with the approximate (showering) weight) By adding the complete [W + jet] ME weight as well the combined weight for W+jet = (approximate + exact) ~ double !! some configurations are counted twice. and the total inclusive cross section is also not well defined When going to W, W+j, W+2j, W+3j, etc, this problem gets worse 

19. Peter SkandsEvent Generator Status 19Matching ►Matching of up to one hard additional jet (since long) PYTHIA-style (reweight shower) HERWIG-style (add separate events from ME: weight = ME-PS) MC@NLO-style (ME-PS subtraction similar to HERWIG, but NLO) ►Matching of generic (multijet) topologies (since a few years) ALPGEN-style (MLM) SHERPA-style (CKKW) ARIADNE-style (Lönnblad-CKKW) PATRIOT-style (Mrenna & Richardson) ►Brand new approaches (still in the oven) Refinements of MC@NLO (Nason) CKKW-style at NLO (Nagy, Soper) SCET approach (based on SCET – Bauer, Schwarz) VINCIA (based on QCD antennae – Giele, Kosower, PS) Evolution

20. Peter SkandsEvent Generator Status 20ALPGEN ►“MLM” matching (Mangano) Simpler but similar in spirit to CKKW ►First generate events the “stupid” way: 1.[W n ] ME + showering 2.[W n+1 ] ME + showering 3.… ►a set of fully showered events, with double counting. To get rid of the excess, accept/reject each event based on: (cone-)cluster showered event  n jets match partons from the ME to the clustered jets If all partons are matched, keep event. Else discard it. ►Virtue: can be done without knowledge of the internal workings of the generator. Only the fully showered final events are needed n inclusive n+1 inclusive n+2 inclusive n exclusive n+1 exclusive n+2 inclusive

21. Peter SkandsEvent Generator Status 21 S. Catani, F. Krauss, R. Kuhn, B.R. Webber, JHEP 0111 (2001) 063 SHERPA, ARIADNE, PATRIOT ►The CKKW algorithm Slices phase space : Matrix Elements for p T > p Tcut and parton shower below 1.[W n ] ME |pT>pTcut * W veto (p Tcut ) + showering pT<pTcut 2.[W n+1 ] ME|pT>pTcut * W veto (p Tcut ) + showering pT<pTcut 3.… [W nmax ] ME|pT>pTcut + showering W veto < 1 is the probability that no parton shower emission happened above p Tcut. Gets rid of double counting: those events that would have caused it are precisely those which do branch above p Tcut Computed using clustered ‘parton shower histories’ on the matrix-element final states. SHERPA uses an approximate analytical formula for W veto. Lönnblad’s ARIADNE-style uses a ‘trial’ or ‘pseudo’ shower, vetoing those events which branch above p Tcut ►A final improvement by Mrenna and Richardson was to require physical flavour and colour flows in the ‘preclustering’ step  PATRIOT database (and HERWIG++?) L. L¨onnblad, JHEP05 (2002) 046 S. Mrenna, P. Richardson, JHEP 0405(2004)040

22. Peter SkandsEvent Generator Status 22MC@NLO Nason’s approach: Generate 1 st shower emission separately  easier matching Avoid negative weights + explicit study of ZZ production Frixione, Nason, Webber, JHEP 0206(2002)029 and 0308(2003)007 JHEP 0411(2004)040 JHEP 0608(2006)077

23. Peter SkandsEvent Generator Status 23 VINCIA ►VINCIA Dipole shower C++ code for gluon showers – running Can evolve in either of 2 different shower evolution variables: pT-ordering (~ ARIADNE) Virtuality-ordering (~ Pythia 6.2 & SHERPA) For each evolution type, an infinite family of radiation functions implemented, all with correct collinear and soft behaviour (= “antenna functions”) ►First parton shower with systematic possibility of variation of shower variable and shower functon  control uncertainties ►To any fixed order, these variations can be absorbed by a new type of matching to matrix elements Dipoles – a dual description of QCD 1 3 2 virtual numerical collider with integrated antennae Giele, Kosower, PS : in progress

24. Peter SkandsEvent Generator Status 24 ►Subtraction method: subtract showering off n-parton state from n+1-parton Matrix Element  subtracted (IR finite) matrix elements. 1.Generate parton-level configurations, with weights given by the subtracted Matrix Elements 2.Shower them using the subtraction function from step 1. ►Similar to HERWIG and MC@NLO approaches (with antenna subtraction instead of Catani-Seymour)  matching at NLO ►Can simultaneously include arbitrarily many tree-level ME’s  multijet matching Combines virtue of CKKW (multijet matching) with that of MC@NLO (NLO matching) Avoids the vices of CKKW (dependence on unphysical clustering and cuts, LO) and MC@NLO (complicated, not applicable beyond one jet) VINCIA-style matching Giele, Kosower, PS : in progress

25. Peter SkandsEvent Generator Status 25 VINCIA Example: H  gg  ggg VINCIA 0.008 Unmatched “soft” |A| 2 VINCIA 0.008 Unmatched “hard” |A| 2 VINCIA 0.008 Matched “soft” |A| 2 VINCIA 0.008 Matched “hard” |A| 2 Giele, Kosower, PS : in progress y 12 y 23 y 12 ►First Branching ~ first order in perturbation theory ►Unmatched shower varied from “soft” to “hard” : soft shower has “radiation hole”. Filled in by matching. radiation hole in high-p T region Long writeup before summer. Then, quarks, ISR, …

26. The Underlying Event Towards a complete picture of hadron collisions

27. Peter SkandsEvent Generator Status 27 ► Domain of fixed order and parton shower calculations: hard partonic scattering, and bremsstrahlung associated with it. ► But hadrons are not elementary ► QCD diverges at low p T ►  multiple perturbative parton-parton collisions should occur ► Normally omitted in explicit perturbative expansions ► + Remnants from the incoming beams ► + additional (non-perturbative) phenomena? Bose-Einstein Correlations Non-perturbative gluon exchanges String-string interactions Interactions with “background” vacuum e.g. 4  4, 3  3, 3  2 Additional Sources of Particle Production

28. Peter SkandsEvent Generator Status 28 A complete model should address… How are the initiators and remnant partons correllated? in impact parameter? in flavour? in x (longitudinal momentum)? in k T (transverse momentum)? in colour (  string topologies!) What does the beam remnant look like? (How) are the showers correlated / intertwined? How does all this hadronize?

29. Peter SkandsEvent Generator Status 29 Interleaved Evolution  Underlying Event (note: interactions correllated in colour: hadronization not independent) Sjöstrand & PS : JHEP03(2004)053, EPJC39(2005)129 multiparton PDFs derived from sum rules Beam remnants Fermi motion / primordial k T Fixed order matrix elements p T -ordered PS matched to ME for W/Z/H/G + jet perturbative “intertwining”? Pythia 6.4

30. Peter SkandsEvent Generator Status 30 ►Fragmentation N ch ~ log(m string ) More strings  more hadrons, but average p T stays same Flat (N ch ) spectrum ~ ‘uncorrellated’ underlying event Underlying Event and Colour Space-time “Area” of string system is large ~ large potential energy “data” models X. Artru, Phys Rept 1983

31. Peter SkandsEvent Generator Status 31 Existing models only for WW  a new toy model for all final states: colour annealing ►Searched for at LEP Major source of W mass uncertainty Most aggressive scenarios excluded But effect still largely uncertain P ~ 10% ►Prompted by CDF data and Rick Field’s studies to reconsider. What do we know? Non-trivial initial QCD vacuum A lot more colour flowing around, not least in the UE String-string interactions? String coalescence? More prominent in hadron-hadron collisions? What is (N ch ) telling us? Implications for Top mass? Implications for LHC? Normal WW Reconnected WW OPAL, Phys.Lett.B453(1999)153 & OPAL, hep-ex0508062 Sjöstrand, Khoze, Phys.Rev.Lett.72(1994)28 & Z. Phys.C62(1994)281 + more … Colour Reconnection (example) Soft Vacuum Fields? String interactions? Size of effect < 1 GeV? Color Reconnections Sandhoff + PS, in Les Houches ’05 SMH Proceedings, hep-ph/0604120

32. Peter SkandsEvent Generator Status 32 Tevatron minimum bias ►Improved Description of Min- Bias ►Effect Still largely uncertain ►Worthwhile to look at top and LHC Colour Annealing: First Results … now interacting with CDF and D0 to try to better measure/constrain effect Last Week: Wicke + PS, hep-ph/0703081 Delta(mtop) ~ 0.5 GeV From infrared effects Delta(mtop) ~ 1 GeV From parton shower Still very primitive model 

33. Outlook Evolution

34. Peter SkandsEvent Generator Status 34 Generators in the LHC era ►MC Generators in state of continuous development Recent explosion of run-your-BSM-through-an-MC-and-see-if-you-can-find- it, sparked by LHC interest. It is at least equally much fun to try to peer deeper into the phenomena of the strong force. Fertile ground both for challenging mathematics, innovative model building, and for early, serious experimental studies. As physics studies pick up, interactions likely to foster more developments  more studies  more developments  … p p http://projects.hepforge.org/pythia6/