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##### Precision QCD at D0: Dijets and Z+jets

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**Precision QCD at D0:**Dijets and Z+jets Gavin Hesketh, Northeastern University 24th July 2009 Wine & Cheese**Precision QCD at D0**G. Hesketh Photon, W, Z etc. parton distribution p Underlying event Hard process parton distribution FSR ISR p hadronization Jet 2 Fragmentation Study fundamental hard parton scattering: - to test understanding of QCD: typically next-to-leading order (NLO) predictions - to constrain the structure of the proton (PDFs) - to search for new interactions Study more complex final states: Z + jets - how well can we calculate and model QCD in these cases? - main background to top, Higgs, SUSY, - do we understand well enough? Precision understanding of QCD is an essential part of the Tevatron program**Overview**G. Hesketh 3 1) Dijets: - foundation: inclusive jet cross section - making further tests of QCD - the components of a measurement - world's best measurements of dijet mass and angles 2) Z + jets: - differential cross sections, including some firsts! - comparisons to pQCD and event generators - what we learn about QCD modeling - and how this is important for finding the Higgs - three publications from August '08 to today!**The Experiment**G. Hesketh y x z 4 Some definitions: - transverse momentum, pT - rapidity y = ½ ln( [E+pz]/[E-pz] ] - pseudo-rapidity = -ln (tan /2)**The Experiment**G. Hesketh y x z 5 Some definitions: - transverse momentum, pT - rapidity y = ½ ln( [E+pz]/[E-pz] ] - pseudo-rapidity = -ln (tan /2) LAr – U calorimeter - “Towers” of cells: - used in triggering and reconstruction - require |z|<50 cm**The Experiment**G. Hesketh y x z 6 Some definitions: - transverse momentum, pT - rapidity y = ½ ln( [E+pz]/[E-pz] ] - pseudo-rapidity = -ln (tan /2) LAr – U calorimeter - “Towers” of cells: - used in triggering and reconstruction - require |z|<50 cm Three distinct regions: - central - ICR - forward**Luminosity**G. Hesketh 7 Tevatron performing excellently: - almost 7fb-1 delivered! - per experiment Thanks to accelerator division! High efficiency: - over 90 % for last two years - thanks to detector experts and shift crews And all data processed - thanks to computing experts! 6.9 6.1**Luminosity**G. Hesketh 8 Tevatron performing excellently: - almost 7fb-1 delivered! - per experiment Thanks to accelerator division! High efficiency: - over 90 % for last two years - thanks to detector experts and shift crews And all data processed - thanks to computing experts! 6.9 6.1 Today Results today all use the first ~1 fb-1 - most results are systematics limited - even with 1 fb-1, able to produce “world's best” results - will highlight areas where adding the full dataset will provide main benefits**Jets**building on inclusive jet measurement: - di-jet mass, angles**Defining a Jet**G. Hesketh 10 Use D0 RunII seeded, iterative, midpoint cone algorithm. Run I algorithm: - draw cone axis around seed (tower) - split/merge after proto-jet finding - recompute axis using ET weighted mean - re-draw cone - iterate until stable. Algorithm sensitive to soft radiation: - infra-red problem.**Defining a Jet**G. Hesketh 11 Use D0 RunII seeded, iterative, midpoint cone algorithm. Run I algorithm: - draw cone axis around seed (tower) - split/merge after proto-jet finding - recompute axis using ET weighted mean - re-draw cone - iterate until stable. Algorithm sensitive to soft radiation: - infra-red problem. D0 Run II algorithm: - add additional seeds between jets - use 4-vectors instead of ET - Jets characterised in terms of pT and y. Improved infra-red stability Algorithm available in fastjet v2.4**Jet Production**G. Hesketh 12 Fundamental process at hadron collider! Inclusive jet cross section constrains PDFs - especially gluon at high x - also probe for quark substructure Tevatron complementary to ep, fixed target LO NLO**Jet Production**G. Hesketh 13 Fundamental process at hadron collider! Inclusive jet cross section constrains PDFs - especially gluon at high x - also probe for quark substructure Tevatron complementary to ep, fixed target Run I measurements left lots of high-x freedom - in Run II, analysed 10x the luminosity - 5x higher cross section at pT = 550 GeV LO NLO**Inclusive Jets**G. Hesketh 14 Benchmark: inclusive jet cross section. - essentially a counting experiment - in bins of jet pT and rapidity Measurements in 6 rapidity bins, - over 9 orders of magnitude - pT up to 650 GeV Most precise measurement to date! For details, please see: - PRL 101062001 - Wine & Cheese, 15th Feb 2008. Compared to NLO prediction - NLO is parton level: - must apply non-perturbative corrections - derived from PYTHIA PRL 101 062001 ('08)**Inclusive Jets**G. Hesketh hep-ph:0901.0002 15 PRL 101 062001 ('08) Experimental unc. smaller than PDF unc. - used in MSTW 2008 PDF fits - Run I jet data excluded from fit - lower gluon at high x Understanding the gluon PDF: - improve sensitivity to new physics - especially important for the LHC**The Next Step**G. Hesketh jet jet 16 Huge amount of work to get this far: - URA Thesis Award for Mikko Voutilainen, User's Meeting 2009 Build on this result by adding some more information: - inclusive jet cross section is essential jet counting - look at the correlations between jets in any given even Parameterise the kinematics with: - dijet mass - yboost = ½ (|y1 + y2|) - y* = ½ |y1 – y2| - or: = exp(2y*) Results for today: 1) dijet mass 2) dijet **Analysis Overview**G. Hesketh 17 From calorimeter energy to a cross section: 1) Selecting events 2) Removing backgrounds 3) Calibrating jets: the jet energy scale 4) Measuring jet resolution 5) Correcting for resolution and efficiency 6) Results**1) Selecting Events**G. Hesketh 18 Triggering on jets is relatively simple: - towers of energy in calorimeter Problem is the rate! - only pT>125 GeV trigger unprescaled We then select events with: - (at least) two reconstructed jets - jet pT > 40 GeV - jet |y| < 2.4 - vertex |z| < 50 cm**2) Backgrounds**G. Hesketh 19 “Physics” backgrounds: - other sources of genuine jets - essentially negligible. “Instrumental” backgrounds: - “fake” jets - spurious jets from noisy cells, EM objects: - remove with jet quality & shape cuts - one main source remained: - bremsstrahlung from cosmic rays Distinct topology: - missing transverse energy balances with jet - reject with simple cut.**3) Calibrating Jets**G. Hesketh 20 Translate calorimeter jet energy to particle jet energy Main tool in energy scale calibration: - pT balance in back-to-back +jet - EM calibration from Z->ee - further corrections for (detector) showering - and for pile-up / min bias overlay**3) Calibrating Jets**G. Hesketh 21 Extend into forward calorimeter with dijets: - account for quark/gluon jet differences - gluon jet response ~ 5% lower. Remarkable achievement: - uncertainties ~ 1-2 % - even into the forward region - 7 years of work! - still dominant uncertainty on jet measurements Note: - we use D0 Run II jet algorithm for detector and particle jets - change jet algorithm: → must re-derive the JES! **4) Jet Resolution**G. Hesketh tail core 22 Jet Resolution measured in dijet events: - attribute pT imbalance to resolution - after accounting for physics effects resolution Jet pT resolution: - smears the measured pT - have to correct for this: unfolding**5) Unfolding**Nevt G. Hesketh 23 Now we correct for detector effects: - resolution and efficiency Jet energy resolution is the main issue: - steeply falling spectrum → migration to higher pT There are several unfolding techniques: - regularized matrix inversion - bin corrections - Bayesian unfolding**5) Unfolding**Nevt G. Hesketh 24 Now we correct for detector effects: - resolution and efficiency Jet energy resolution is the main issue: - steeply falling spectrum → migration to higher pT There are several unfolding techniques: - regularized matrix inversion - bin corrections - Bayesian unfolding Mostly using a bin-correction method: 1) take a model of the true distribution - mathematical ansatz, or Monte Carlo 2) smear with data resolutions - full detector simulation, or parameterized 3) optimise the model: - fit the ansatz, or re-weight the Monte Carlo 4) take the ratio of true / smeared in each bin - apply to data We also use regularised matrix inversion on some results.**5) Unfolding Dijet Mass**G. Hesketh 25 D0 Preliminary Unfolding corrections: - typically 1-15 % - combined with jet efficiencies Efficiencies from “tag and probe”: - typically ~ 98 % Calorimeter jet Charged track jet ?**6) Result: Di-jet Mass**G. Hesketh 26 Measurement of di-jet mass: - For R=0.7 cone jets, jet pT > 40 GeV - 6 jet rapidity regions 0 < |y| < 2.4 - extend measured region - masses up to 1.4 TeV! Compare to NLO: - prediction from fastNLO (arXiv:hep-ph/0609285) - using MSTW2008 NLO PDF - = ½ (pT1 + pT2) Apply non-perturbative corrections: - parton level theory → particle level - derive from PYTHIA - typically ~5 %**6) Result: Di-jet Mass**G. Hesketh 27 NLO + MSTW2008 agrees within systematics - note the PDF uncertainty and data are now correlated! Is there more information on PDFs here than inclusive jets? - require most forward jet to lie in |y| region - so, possibly... Preliminary result, Spring 2009**Tevatron vs LHC**G. Hesketh 28 At the LHC: - cross section vs pT obviously much larger BUT cross section vs x significantly smaller! e.g. for |y|<0.4, factor of 200 at x = 0.5 D0 results with 0.7 fb-1 → need 140 fb-1 at LHC Further, problem of steeply falling spectrum: at D0, 1% error on jet energy calibration → 5 - 10% error on central →10 - 25% error on forward At LHC: - need excellent jet energy scale - out to very high pT Expect Tevatron to dominate high-x gluon PDF for some years!**New Physics?**large y G. Hesketh small y 29 Dijet mass is an excellent test of pQCD and PDFs. Can also search for new physics! - resonance decaying to dijets - excess at high mass due to new interactions - studies ongoing Other kinematics are also sensitive to new physics - jet angles are a nice candidate Focus on = exp(|y1 - y2|) - in massless, 2→2 limit: - interaction with different kinematics to QCD → different dijet distribution - any deviation from QCD prediction → new physics!**Di-jet **G. Hesketh 30 Here, we care primarily about the shape: - measure normalised distribution - for jet with pT > 40 GeV - |y| < 2.4 - in bins of dijet mass We have all the components in place: - jet triggers and reconstruction - background rejection - jet energy scale and resolution Jet rapidity very well measured: - migrations mainly in jet pT - cause migrations between dijet mass bins. Derive unfolding corrections from PYTHIA - re-weighted to data - with parameterised detector simulation**Di-jet **G. Hesketh 31 Submitted to PRL; arXiv:0906.4819 New measurement of di-jet - most precise at a hadron collider - in 11 bins of di-jet mass - first measurement above 1 TeV - most sensitive to new physics Result is statistics limited! - add more luminosity in the future Compared to NLO pQCD: - very small theory uncertainty - width of the red line! An early analysis at LHC? - not really! - in a given mass bin, you have - low : central jets - high : forward jets - need good understanding of: - jet energy scale, resolution - correlations in pT and rapidity**Limit Setting**G. Hesketh 32 Submitted to PRL; arXiv:0906.4819 QCD prediction matches the data so far. Extract limits on three models: - quark compositeness: P. Chiappetta, M.Perrottet, PLB 253: 489 (1991) - ADD large extra dimensions: D. Atwood, S. Bar-Shalom, A.Soni, PRD 62 (2000) - TeV-1 scale extra dimensions: K. Cheung, G.Landsberg, PRD 65 (2002) Define transformed scale:= 1/2 (QC), 1/M4 (ADD LED), 1/M2 (TeV-1 ED) - extract 95% Bayesian limits in assuming prior flat in and 2 Best direct limits on these models! - provide full correlation tables for further comparisons**Z Production**q q g Z G. Hesketh 34 Use leptonic (ee, ) Z decays as probe of QCD - high Q2 (~MZ or MW) - very small backgrounds, right down to pT ~0! Concentrate on high pT final states: Z + jets - regime of perturbative QCD pQCD: - LO W/Z + 1 - 6 partons - NLO W/Z + 1, 2 (MCFM) - new NLO W+3 (Rocket, Blackhat+SHERPA)**Z Production**q q g Z G. Hesketh 35 Use leptonic (ee, ) Z decays as probe of QCD - high Q2 (~MZ or MW) - very small backgrounds, right down to pT ~0! Concentrate on high pT final states: Z + jets - regime of perturbative QCD pQCD: - LO W/Z + 1 - 6 partons - NLO W/Z + 1, 2 (MCFM) - new NLO W+3 (Rocket, Blackhat+SHERPA) Event generators: - LO 2 ->1, 2 + parton shower - PYTHIA, HERWIG - LO 2 -> 1-6 + (vetoed) parton shower - ALPGEN, SHERPA These generators are the main Tevatron and LHC tools, - but, leading order → large uncertainties - must to be tuned to data!**Z + Jets**G. Hesketh 36 The other motivation: - (W or) Z + jets is the main background to top, higgs, some SUSY models. - example: associated Higgs production: WH→lbb, ZH→llbb ALPGEN is the main simulation used at D0. But: - problems describing some distributions - re-weight based on detector level information, assign systematic Ideally: find a generator that describes data “out of the box” - can some tuning fix our problems? To provide useful inputs for the generator community: - need fully corrected distributions.**Experimental Issues**Z→ G. Hesketh 37 Triggering and event selection: - trigger & select high pT (15 - 20 GeV) Z “Physics” Backgrounds: - cosmic rays (, negligible) - Z→ , WZ, WW, top pair (0.5% - 1%) Z “instrumental” backgrounds: - high EM fraction jets (~1%) - reject with shower shape cuts - semi-leptonic decays (< ~0.5%) - reject with isolation criteria M**Experimental Issues**Z→ G. Hesketh 38 Triggering and event selection: - trigger & select high pT (15 - 20 GeV) Z “Physics” Backgrounds: - cosmic rays (, negligible) - Z→ , WZ, WW, top pair (0.5% - 1%) Z “instrumental” backgrounds: - high EM fraction jets (~1%) - reject with shower shape cuts - semi-leptonic decays (< ~0.5%) - reject with isolation criteria Measure lepton efficiencies using “tag and probe” - with Z events Measure lepton resolution / energy scale: - width and position of Z mass peak Unfold using bin-corrections method, and matrix inversion**Z + Jets**G. Hesketh 39 Measurement of 1st, 2nd and 3rd jet pT in Z events: - Z→ee, jet pT > 20 GeV, jet |y|< 2.5. - normalize to inclusive Z production (cancel some uncertainties) PLB 678, 45 (2009) Leading jet in Z + jet + X The differential cross section, normalised to inclusive Z production NLO: MCFM - CTEQ6.6M PDF - R2 = F2 = pTZ2 + MZ2 Data / NLO LO / NLO + scale uncertainties NLO scale uncertainties**Z + Jets**G. Hesketh 40 Measurement of 1st, 2nd and 3rd jet pT in Z events: - Z→ee, jet pT > 20 GeV, jet |y|< 2.5. - normalize to inclusive Z production (cancel some uncertainties) PLB 678, 45 (2009) Leading jet in Z + jet + X Second jet in Z + 2jet + X**Z + Jets**G. Hesketh 41 Measurement of 1st, 2nd and 3rd jet pT in Z events: - Z→ee, jet pT > 20 GeV, jet |y|< 2.5. - normalize to inclusive Z production (cancel some uncertainties) PLB 678, 45 (2009) Leading jet in Z + jet + X Second jet in Z + 2jet + X Third jet in Z + 3jet + X**Z + Jets**G. Hesketh 42 Now the event generators: PLB 678, 45 (2009) Leading jet in Z + jet + X Parton Shower MC: - PYTHIA pT ordered shower / NLO - PYTHIA Q2 ordered shower / NLO - HERWIG / NLO Matrix element + Parton Shower MC: - ALPGEN+PYTHIA / NLO + scale unc. F2 = pTZ2 + MZ2 - SHERPA / NLO**Z + Jets**G. Hesketh 43 Including the higher order matrix elements pays off for second, third jet PLB 678, 45 (2009) Leading jet in Z + jet + X Second jet in Z + 2jet + X**Z + Jets**G. Hesketh 44 Including the higher order matrix elements pays off for second, third jet Treating the scale choice as a tuneable parameter: - best description from ALPGEN with lower scale (default: F2 = pTZ2 + MZ2). PLB 678, 45 (2009) Leading jet in Z + jet + X Second jet in Z + 2jet + X Third jet in Z + 3jet + X**Z + Jet + X**jet Z jet G. Hesketh 45 Take a more detailed look at Z(→) + ≥1 jet jet Z pT in Z + jet + X PLB 669, 278 (2008) Z Event Generators should handle low and high pT: - in fact, a nice region to use for tuning! Upgrade the theory comparisons: - MCFM v5.4, NLO and LO, with MSTW 2008 PDF - PYTHIA 6.420 - new “Perugia” tune for pT ordered shower - using the “modified leading order” MRST2007 PDF - SHERPA 1.1.3 (updated tune) - ALPGEN was the best for jet pT, add more options: - shower with HERWIG - shower with PYTHIA Q2 ordered (tune QW) - shower with PYTHIA pT ordered (Perugia tune) - thanks to Steve Mrenna**Z + Jet + X**Z + Jet G. Hesketh 46 Take a more detailed look at Z(→) + ≥1 jet Z pT in Z + jet + X PLB 669, 278 (2008) PYTHIA pT ordered - new “Perugia” tune - MRST07 LO* PDF PYTHIA Q2 ordered HERWIG ALPGEN + PYTHIA pT ALPGEN + PYTHIA Q2 ALPGEN + HERWIG**Z + Jet + X**G. Hesketh 47 Take a more detailed look at Z(→) + ≥1 jet - we also need good description of jet angles: look at leading jet rapidity Jet rapidity in Z + jet + X PLB 669, 278 (2008) PYTHIA pT ordered - new “Perugia” tune - MRST07 LO* PDF PYTHIA Q2 ordered HERWIG ALPGEN + PYTHIA pT ALPGEN + PYTHIA Q2 ALPGEN + HERWIG**Z + Jet + X**Z + Jet Z G. Hesketh 48 Further constrain the kinematics: - look at angles between the Z and leading jet - first measurements at a hadron collider of: - (Z, jet), |y (Z, jet)|, |yboost(Z, jet)| = |½ (y1 +y2)| Take a closer look at (Z, jet): - trivial in the absence of additional jets - LO pQCD corrections come from Z + 2 jets (at LO) - NLO pQCD corrections from Z + 3 jets (at LO) - thanks to MCFM authors for making this possible! ie with (Z, jet), we can test all current pQCD predictions! - without systematics associated with jet reconstruction**Z + Jet + X**Z + Jet Z G. Hesketh 49 Further constrain the kinematics: - look at angles between the Z and leading jet - first measurements at a hadron collider of: - (Z, jet), |y (Z, jet)|, |yboost(Z, jet)| = |½ (y1 +y2)| Take a closer look at (Z, jet): - trivial in the absence of additional jets - LO pQCD corrections come from Z + 2 jets (at LO) - NLO pQCD corrections from Z + 3 jets (at LO) - thanks to MCFM authors for making this possible! ie with (Z, jet), we can test all current pQCD predictions! - without systematics associated with jet reconstruction Note: to get a meaningful measurement of (Z), require: - Z pT > 25 GeV - Z pT > 45 GeV Below this, dominated by muon pT resolution |y (Z, jet)|, |yboost(Z, jet)| dominated by PDF contribution - but still sensitive to additional radiation**y(Z, jet)**G. Hesketh 50 First measurement of y(Z, jet) ! - Z→, |y|<1.7, pTZ > 25 GeV - jet pT>20 GeV, |jet y| < 2.8 Submitted to PLB today! arXiv:0907.4286 PYTHIA pT ordered - new “Perugia” tune - MRST07 LO* PDF PYTHIA Q2 ordered HERWIG ALPGEN + PYTHIA pT ALPGEN + PYTHIA Q2 ALPGEN + HERWIG