Vector boson + jets at NLO vs. LHC data

Transcription

1 Vector boson + jets at NLO vs. LHC data Lance Dixon (SLAC) for the BlackHat collaboration Z. Bern, LD, G. Diana, F. Febres Cordero, S. Höche, H. Ita, D. Kosower, D. Maître, K. Ozeren LHC Theory Workshop University of Melbourne - July 4, 2012

2 New physics at the LHC is a riddle, wrapped in a mystery, inside an enigma; but perhaps there is a key. -W. Churchill L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

3 LHC Data Dominated by Jets Jets from quarks and gluons. q,g from decay of new particles? Or from old QCD? new physics Every process shown also with one more jet at ~ 1/5 the rate Need accurate production rates for X + 1,2,3, jets in Standard Model L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

4 Classic SUSY dark matter signature Heavy colored particles decay rapidly to stable Weakly Interacting Massive Particle (WIMP = LSP) plus jets Missing transverse energy MET + 4 jets L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

5 Irreducible Standard Model Background MET + 4 jets from pp Z + 4 jets, Z neutrinos Neutrinos escape detector Now available at Next to Leading Order, greatly reducing theoretical uncertainties n n State of art for Z + 4 jets was based on Leading Order (LO) approximation in QCD normalization uncertain L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, LO NLO n n

6 NLO = Loops NLO cross section needs Feynman diagrams with exactly one closed loop Textbook methods quickly fail, even with very powerful computers NLO also needs tree-level amplitudes with one more parton Both terms infinite combine in dim. reg. to get a finite result NLO 1 loop tree + 1 parton One-loop amplitudes were the bottleneck for a long time L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

7 A Better Way to Compute? Want loop-level scattering amplitudes for many ultra-relativistic ( massless ) particles especially quarks and gluons of QCD Feynman diagrams, while very general and powerful, are not optimized for these processes There are much more efficient methods L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

8 Just one QCD loop can be a challenge pp W + n jets (just amplitudes with most gluons) # of jets # 1-loop Feynman diagrams Current limit with Feynman diagrams Current limit with on-shell methods L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

9 Branch cut information Generalized Unitarity (One-loop Plasticity) Ordinary unitarity: put 2 particles on shell Generalized unitarity: put 3 or 4 particles on shell Trees recycled into loops! L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

10 One-Loop Amplitude Decomposition Bern, LD, Dunbar, Kosower (1994) Missing from the old, nonperturbative analytic S-matrix coefficients can be determined from products of trees using (generalized) unitarity Known functions (integrals), same for all amplitudes rational part; from D-dimensional trees, or recursively L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

11 Recent progress on V + jets at NLO MCFM: V + 0,1,2 jets Campbell, Ellis, hep-ph/ Rocket: W + 3 jets Ellis, Melnikov, Zanderighi, , Blackhat+Sherpa: Berger, Bern, LD, Diana, Febres Cordero, Forde, Gleisberg, Höche, Ita, Kosower, Maître, Ozeren W + 3 jets , Z + 3 jets W + 4 jets Z + 4 jets W + 5 jets 12mm.nnnn Could try to use such predictions directly for backgrounds to experimental searches. However, it is generally safer to use data-driven techniques L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

12 NLO pp W + 4 jets Berger et al., First hadron collider process known at NLO with 5 objects in final state. Also important SUSY background. L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

13 NLO pp Z + 4 jets, and ratio to W ± Ita et al L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

14 NLO pp W + 5 jets also feasible + + PRELIMINARY leading color BH+Sherpa L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

15 vs. LHC data L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

16 NLO pp Z + 1,2,3,4 jets vs. ATLAS 2010 data ATLAS Analysis in progress with full 2011 data set: > 100 times the 2010 data L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

17 NLO pp W + 1,2,3,4 jets vs. ATLAS 2010 data ATLAS NLO undershoots badly for W + 1 jet production dominated by W + 2 parton configurations. Theory can be improved here: Rubin, Salam, Sapeta L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

18 Issues with forward jets? ATLAS Large x gluon too large? L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

19 Also forward jet issues at Tevatron PLHC Vancouver Note that probability of emitting another jet seems OK Important experimentally for assessing effects of central jet veto on backgrounds, in search for Higgs in vector boson fusion L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

20 Pure QCD: pp 4 jets vs. ATLAS data ATLAS jet events might hide pair production of 2 colored particles, each decaying to a pair of jets [e.g. talk by B. Dobrescu] Detailed study of multi-jet QCD dynamics may help understand other channels L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

21 Pure QCD Jet Production Ratio 3/2 ratio agrees well with ATLAS data even in regions where it s quite large Looking forward to measurement of 4/3 ratio ATLAS L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

22 Fixed order vs. Monte Carlo Previous plots NLO but fixed-order, few partons: no model of long-distance effects included; cannot pass through a detector simulation Methods available for matching NLO parton-level results to parton showers, with NLO accuracy: MC@NLO Frixione, Webber (2002) + SHERPA implementation POWHEG Nason (2004); Frixione, Nason, Oleari (2007) Recently implemented for increasingly complex final states L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

23 NLO MC for W + 1,2,3 jets vs. ATLAS data Höche et al., L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,

24 Conclusions Thanks to new methods for computing loop amplitudes, lots of NLO (fixed order, parton-level) results now available for comparison with LHC data, in particular: W/Z + 1,2,3,4,5 jets 4 jets Except for some issues at large jet rapidity, agrees quite well with 2010 LHC data ATLAS analyses of both W and Z + 1,2,3,4 jets, & pure QCD 4 jets So far, the NLO results for the most complex processes are only at parton level, not yet embedded in a full Monte Carlo, like MC@NLO, POWHEG, or SHERPA. But this will happen before long already done up to W + 3 jets! Looking forward to learning even more from comparisons to 2011 LHC data from both CMS and ATLAS L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4,