ATLAS 2.76 TeV inclusive jet measurement and its PDF impact A M Cooper-Sarkar PDF4LHC Durham Sep 26th 2012 In 2011, 0.20 pb-1 of data were taken at √s = 2.76 TeV, ATLAS-CONF-2012-128 This is plenty for an inclusive jet cross section measurement to be systematics rather than statistics limited The correlated systematic uncertainties of this measurement are fully correlated to those of the published inclusive jet cross section measurement made at √s = 7 TeV with 36pb-1 of 2010 data (arXiv:1112.6297) Thus the cross section ratios of these measurements has a significantly reduced uncertainty The ratio measurement has some PDF impact

The anti-kt algorithm is used to identify jets with two jet distance parameters, R = 0.4 and R = 0.6, R=0.6. The measurement is made in the kinematic regions 20 ≤ pT <430 GeV and |y| < 4.4 at √s = 2.76 TeV

The jet reconstruction procedures and calibration factors are the same as those used for the 7TeV measurement. Thus the systematic uncertainties on this jet measurement are evaluated considering the same uncertainty sources as for the 7 TeV measurement (mostly!)

Compare the dominant experimental uncertainties for the 2 Compare the dominant experimental uncertainties for the 2.76 TeV jet cross sections To those on the ratio of the 2.76 to7 TeV jet cross sections

Also compare the uncertainties on NLO pQCD predictions for the 2 Also compare the uncertainties on NLO pQCD predictions for the 2.76 jet cross-sections – from scale choice, ΔαS ± 0.002 and CT10 PDF uncertainties- to the uncertainties on NLO pQCD predictions for the ratio of 2.76 jet cross sections to 7 TeV jet cross sections . The PDF uncertainties are still substantial because measurements at different centre of mass energies probe different x and Q2 values for the same pt and y ranges . Thus increased sensitivity to the PDFs is expected when the two sets of jet data are analysed together.

First consider the 2.76 jet cross sections in ratio to predictions using CT10 PDFs and compared to predictions using MSTW2008, NNPDF2.1 and HERAPDF1.5. Predictions are done using NLOjet++ interfaced to Applgrid R=0.6 is shown (R=0.4 in back-up) Systematic uncertainties of the data are large so it is not easy to assess PDF impact from such a plot

Now consider the ratio of the 2 Now consider the ratio of the 2.76 jet cross-sections to the 7TeV jet cross sections in ratio to the CT10 predictions for this ratio and compared to the predictions of MSTW2008, NNPDF2.1 and HERAPDF1.5 R=0.6 is shown (R=0.4 in back-up). The experimental systematic uncertainty is reduced and is generally smaller than the theory uncertainty. There is potential for PDF discrimination

So is there any PDF discrimination? The inclusive jet pt spectrum at low and moderate pt should be sensitive to the gluon PDF. How to evaluate this? The impact of any one data set on a PDF set will depend on the details of how that PDF set is determined , i.e. What other data sets are used What weights (if any) are applied to data sets What Δχ2 tolerances are chosen to set the uncertainties for each eigenvector We investigated the impact of these jet data by inputting them io an NLO QCD PDF fit with just HERA-1 combined data. We use Δχ2 =1 to set 68% uncertainties This is a simple choice to include only a minimal data set which are necessary for the determination of PDFs. We assess only the impact on experimental uncertainties not the possible further impact on model uncertainties or parametrisation uncertainties.

Fit Details Use HERAfitter open source code Combined data on DIS inclusive cross sections e+/e-/NC/CC from HERA-1 Plus ATLAS 2.76 and 7 TeV inclusive jet cross sections QCDNUM for NLO DGLAP evolution and light quark coefficient functions Thorne-Roberts Variable Flavour Number for heavy quark coefficient functions Mc=1.4, Mb=4.75 GeV, αS(MZ) = 0.1176 Q2min=3.5 GeV2 for inclusive cross sections, ptmin=45 GeV for jet data sets NLOpQCD predictions for jets from NLOjet++ interfaced to Applgrid Non-perturbative corrections for hadronisation ad underlying event. Q20=1.9 GeV2 and the PDF parametrisation at Q20 is 13 parameters g

A fit is first made using only the HERA-1 data, and this is then compared with a fit which includes the HERA-1 data and the ATLAS jet data sets See above for the comparison of the predictions to the 2.76 jet cross sections. The fit which includes these data follows the shape of the jet data very well. The χ2 for both the jet data sets and the HERA data are very good (χ2 ~30 for 40 points at 2.76 TeV and χ2~45 for 76 points at 7 TeV)

The generic form for the χ2 is Now consider this comparison for the ratio of the 2.76 and 7 TeV jet cross sections.. The generic form for the χ2 is The fits determine the PDF parameters and they also determine shifts for the correlated systematic uncertainties of the data. These shifts are applied to the theoretical predictions. Typically they are ~0.5 of a standard deviation. The normalisations of the data sets are not correlated to each other and their shifts are detrmined as +1.0 (+1.4) σ for 7 TeV and -0.7 (+0.2) σ for 2.76 TeV data for R=0.6 (0.4)

Finally Compare a the gluon and total sea PDFs for a fit using just HERA data and a fit using HERA+ ATLAS 2.76 and 7 TeV jet data. Experimental uncertainties only- no model and parametrisation uncertainties Gluon becomes harder, Sea slightly softer at high-x. Uncertainties on gluon are reduced. The impact is stronger than if either the 7 TeV or the 2.76 TeV jets are used alone.

SUMMARY In 2011 0.20 pb-1 of data were taken at √s = 2.76 TeV ATLAS-CONF-2012-128 This is plenty for an inclusive jet cross section measurement to be systematics rather than statistics limited The correlated systematic uncertainties of this measurement are fully correlated to those of the published inclusive jet cross section measurement made at √s = 7 TeV with 36pb-1 of 2010 data (arXiv:1112.6297) Thus the cross section ratios of these measurements has a significantly reduced uncertainty The ratio measurement has some PDF impact

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