jet-jet angular distributions and search for quark substructure in p-p collisions at 1.96 tev
DESCRIPTION
Jet-jet angular distributions and search for quark substructure in p-p collisions at 1.96 TeV. Lee Pondrom, U. of Wisconsin, for the CDF Collaboration XIX International Baldin Seminar September 29-October 4, 2008. Tevatron Run2 Collider operations. - PowerPoint PPT PresentationTRANSCRIPT
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Jet-jet angular distributions and search for quark substructure in
p-p collisions at 1.96 TeV Lee Pondrom, U. of Wisconsin,
for the CDF CollaborationXIX International Baldin SeminarSeptember 29-October 4, 2008
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Tevatron Run2 Collider operations
• About 5 fb-1 delivered, and 4 fb-1 recorded each by CDF and D0.
• About 2 fb-1 delivered in calendar 2008.
• Instantaneous luminosity record 315 E30.
• Initial pbars 3E12 at 980 GeV.
• Run to last one or two more years.
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Tevatron collider operations
Store 6434 Monday Sept 22
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Data Sample
• 1.1 fb ¹ integrated luminosity
• 12 million jet100 triggers
• 10 nb constant trigger cross section
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Minimal cuts
• ||<2
• |Zvertex|<60 cm
• Missing ET significance < 5 GeV
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Typical dijet event display
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QCD 22 angular distributions
• The formulas of Combridge for q+qq+q,
q+qbarq+qbar, q+qbarg+g, q+gq+g, g+gg+g, and g+gq+qbar give angular distributions which resemble Rutherford’s formula dd~1/sin4(*/2).
Rutherford’s formula is flat in exp(|2|)
Where ’s refer to the two leading jets
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Jet-jet angular distribution and quark substructure
• Quark substructure effective contact color singlet Lagrangian of Eichten, et al is:
• L = ±(g²/2Λ²(LLLL
• Looks just like muon decay. Affects only the u and d quarks. Color singlet means that some diagrams have no interference term.
• g²/4 = 1; strength of the interaction ~(ŝ/²)²• This measurement is not sensitive to the interference
term.
_ _ _
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Effect of quark substructure
• The quark substructure Lagrangian is basically isotropic, so the angular distribution near *=, or is most sensitive to .
• The ET distribution also depends on , but is more sensitive to the jet energy scale than the angular distribution in a given mass bin.
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treatment of the data
• Divide the data into four bins in jet-jet invariant mass, using the two highest ET
jets in the event. Do not look for third jets.
• Each bin is 100 GeV wide, starting at 550-650 GeV, and ending at 850-950 GeV.
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Monte Carlo predicts the expected QCD distributions, and the effects of quark substructure
• The MC program used is Pythia. • Pythia generates the QCD event at the
‘hadron level’, without the CDF detector simulation, via a multistep process involving ISR, 22,FSR, and parton fragmentation.
• Hadron level events are then subject to the full CDF detector simulation, and analyzed with the same code as data.
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Pythia Angular distributions compared to CDF data
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Pythia angular distributions compared to CDF data
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Pythia simulation fits to dataa1pT
2+a2ŝ
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Pythia simulation fits to data
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Pythia Monte Carlo Simulation of quark substructure
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Pythia Monte Carlo Simulation of quark substructure
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Distribution sensitivity to
• A ratio method was used to measure the effect of on the angular distribution in a given mass bin.
• Define R=(1<<10)/(15<<25).• Using Pythia for the dependence, plot R()/R()
versus (mass)4, where R() means no quark substructure.
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Dependence of the ratios vs (mass)4 on the parameter
Systematic
uncertainties included
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Sensitivity of the slope to the quark substructure parameter
Limit is dominated by systematic uncertaities
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Systematics
• There are many adjustable parameters in the comparison of data to Monte Carlo simulation.
• Is it possible to adjust the Monte Carlo to mask the presence of new physics in the data?
• The answer is yes. The study of systematics should give the degree of flexibility inherent in the comparison.
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To obtain a limit we must understand the systematics
• Sensitivity to the choice of the parton distribution functions. We are looking at high mass dijets, searching for quark substructure, so the most important pdf’s are proton valence valence.
• The first study compared CTEQ and MRST.
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MRST compared to CTEQ
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MRST compared to CTEQ
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Systematics of the pdf’s
• MRSTLO ‘high s’ and CTEQ5L predict the same angular distributions with no quark substructure.
• A new method for evaluating uncertainties from the pdf’s, using ‘vectors’ which represent uncertainties coming from the input experimental data.
• Preliminary studies of the vectors in CTEQ6 indicate small effects on the ratios.
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Systematic studies continued
• Choice of Q2. Here the angular distributions differ. Vary the mix of ŝ and pT
2 by 1.
• The jet energy scale. Use the utility to vary the jet energy corrections by ±1. The high mass jet-jet cross section depends on the jet energy corrections.
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Procedure
• Calculate R for three MC samples: best fit, +1 and -1, varying the choice of Q2.
• Do a simple average <R>=(R1+R2+R3)/3
• Calculate R for three data samples:level7 jetEcorrections, +1 and -1.
• Again do a simple average <Rd>=(R1d+R2d+R3d)/3
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Systematic uncertainties
• The systematics are included in the uncertainty in each ratio by summing the deviations from the mean: dR² = i=1,3(Ri-<R>)²/2 for the MC, and similarly for dR²d.
• Then the final ratios Rd/R are calculated for each mass bin, and plotted vs (mass)4
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Summary
• The ratio R=(110)/(1525) shows a linear dependence vs x=(mass)4 , with a slope which increases with increasing .
• The data have a slope which is slightly negative: dR/dx = -0.160.08. This result is unphysical.
• To set a limit, we use the Feldman Cousins method (PRD 57,3873 (1998)).
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Feldman Cousins method
• The method is based on physically allowed results versus experimental results, which can be unphysical.
• The uncertainties must be known, but not the result.
• For a set of allowed results, generate all possible outcomes, using the uncertainties.
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Feldman-Cousins plot
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Final limits
• By integration, 95% and 68% confidence contours can be extracted from this plot.
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Feldman Cousins limit contours
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Limits from the plot
• From the intersection of the measured slope with the confidence level contours, we conclude:
• Slope<0.24 95% confidence
• Slope<0.06 68% confidence
• Expected slope limit for zero slope result (based on these experimental uncertainties) slope<0.35
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Sensitivity of the slope to the quark substructure parameter
Limit is dominated by systematic uncertaities
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Conclusions
• To interpret these slopes as lower limits on the quark substructure parameter , we must rely on Pythia Monte Carlo simulation, which gives the sensitivity of the slope of the angular distribution ratio to .
• 95% confidence >2.4 TeV• 68% confidence >3.5 TeV• 95% confidence expected result >2.2 TeV
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Дополнительный материал
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Requested material
• Three topics are covered. Not part of the blessing.
• 1. Extra jetEnergy uncertainty in the plug
• 2. Hadron level corrections from Pythia
• 3. Trigger threshold at 100 GeV in the Monte Carlo
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Check of the effect of the extra plug uncertainties
• The normal level7 jetEcorrections package was run on 5M data jet100 events
• The supplied uncertainty formula was applied only to those jets (1.1<|det|<2.1)
• Central jets were not affected.
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Additional jet energy uncertainties
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Conclusion of the plug uncertainty exercise
• Hardly any visible effect.
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Hadron level correctionsnotice how flat the had level
distributions are
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Monte Carlo ratios had/detector
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Trigger threshold in the Monte Carlo simulation
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Monte Carlo trigger
• The conclusion here is that, while the distribution of the Monte Carlo in the 550-650 GeV mass bin does not exactly match the data, and the resulting 2 is bad, the discrepancy is not due to the trigger turn on.
• There are small discrepancies in all of the plots, and each bin in has over 1000 events.
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CTEQ6 and the vectors
• Only CTEQ6 has the vectors. Earlier versions, like the default CTEQ5L used here, do not.
• There are 40 vectors, in pairs up and down. They are linearly independent
• It is not obvious which vectors affect specific pdf’s – like up quarks.
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CTEQ6 and the vectors cont’d
• Preliminary studies with the full Pythia Monte Carlo, but without the CDF detector simulation, show that the vector effect is small.
• Conclusion: The lack of knowledge of the pdf’s is not an important contributor to the systematic uncertainty.
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This is the analysis for which blessing is requested
• The following slides were requested after the June 12 presentation.
• Correction of the data to the hadron level is necessary to compare to NLOJet++, a subject which we prefer to postpone at the present time. The analysis for preblessing compares data as taken with fully simulated Monte Carlo.
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Further studies with Nlojet++
• NLO QCD C++ code written by Zoltan Nagy (arXiv:hep-ph/0110315)
• Downloadable from CERN• Born+nlo calculations with 5E9 events• Three partons in the final state, pairs
clustered in cone 0.7 merged to one ‘jet’.• Hard scale 0=ETave and 0=mjj match
Pythia choices
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Data hadron level corrected compared to Pythia for Q2=pT
2 +ŝ
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Pythia fit to the data corrected to the hadron level.
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=3 TeV Pythia fits to data
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Nlojet++ angular distributions for two choices of hard scale
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Nlojet++ compared to Pythia
• Slightly different pdf’s:CTEQ5L for Pythia and CTEQ6 for Nlojet++
• The dependence on the hard scale is much reduced in Nlojet++, where ETave and mjj have almost the same shapes.
• The data follow 0=pT (Pythia), but either choice for 0 works with Nlojet++
• This justifies the procedure used earlier,where the variation in Q2 was limited in systematics
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Nlojet++ fits to the data-about 50-50 ETave and mjj
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For Nlojet++ define R=(1->10)/(10->20)
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Switch on quark substructure in Pythia.
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R from Pythia with quark substructure
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2 vs -4 for R(data) using linear fits from Pythia
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Conclusions
• Nlojet++ confirms the limited variation of Q2 in Pythia in systematics studies.
• Jet energy corrections systematics need another look.
• Hadron level jet100 data are consistent with nlo QCD predictions.
• Pythia must be used to extract , and the limit for (u,d,s) quarks will be near 3 TeV.
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Vertex and MET before cuts
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Data kinematic distributions
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More data kinematic distributions
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Monte Carlo predicts the expected QCD distributions, and the effects of quark substructure
• The MC program used is Pythia. • Pythia generates the QCD event at the
‘hadron level’, without the CDF detector simulation, via a multistep process involving ISR, 22,FSR, and parton fragmentation.
• Hadron level events are then subject to the full CDF detector simulation, and analyzed with the same code as data.
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Modifications to the default QCD monte carlo
• mstp(32) Q² flag from 8-pt² to 4-ŝ• msel from 1 to 0 allows user to switch on new physics• msub(381) to msub(388) standard QCD 2
subprocesses• ckin(1)=400. sets ŝ>400 GeV• ckin(3)=90. sets pt>90 GeV/c• itcm(5)=1 switches on quark substructure• Rtcm(41)=Λ in GeV• rtcm(42)=±1 sign of interference term
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MC cut on CM energy
• The requirement ckin(1)=400 was made to save MC disk space and execution time.
• The QCD default runs with ckin(3)=200 give similar mass distributions, but distort the angular distributions, so are not suitable.
• An overall factor of five is gained in total number of events vs events at high mass.
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MC pT>90GeV Q²=pT² CTEQ5L
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Data and Pythia ET and mass
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Fits of the MC to data 48 bins
• Data(k)=a1pT2(k)+a2ŝ(k) Mass
GeV
Events
dataa1 a2 ²
550-
650
165926 1.50±
0.04
0.49±
0.04
87.8
650-750
54404 1.52±
0.07
0.46±
0.07
28.2
750-850
16112 1.40±
0.13
0.54±
0.12
57.9
850-950
4581 1.14±
0.17
0.77±
0.17
39.7
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R ratios vs jet-jet (mass)4
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Plot the slope vs 1/4
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Q² not correlated with PDF’s
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Q² dependence study
Mass
GeV
a1 up a2 dn a1 dn a2 up ²
600 1.58 .41 1.42 .57 93
700 1.66 .33 1.39 .59 32
800 1.66 .3 1.14 .78 62
900 1.48 .43 .8 1.11 44
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Plots varying the Q² fit
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Jet energy corrections
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Angular distributions ±1σ JetEcorr
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Monte Carlo R ratios for systematics
Mass bin
GeV
R pT² dn 2 R best fit R pT² up 2
600 0.994±0.006 0.978±0.008 0.963±0.007
700 0.849±0.011 0.825±0.013 0.802±0.012
800 0.846±0.020 0.803±0.023 0.762±0.021
900 0.829±0.036 0.779±0.042 0.731±0.036
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Jet100 data varying jetEcorr for systematics
Mass bin GeV
Up 1 Level7 jetEcor
Down 1
600 1.050±0.006 0.967±0.006 0.905±0.006
700 0.839±0.009 0.817±0.010 0.788±0.011
800 0.788±0.016 0.766±0.018 0.742±0.020
900 0.756±0.029 0.711±0.031 0.696±0.036
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=2TeV Pythia fits to data