Pt spectra for the First Pass and Second Pass jets
3.2 Some Extrapolations of Tevatron Measurements and the Impact on Heavy Quark PDFs Contributed by: Campanelli
3.2 Some Extrapolations of Tevatron Measurements and the Impact on Heavy Quark PDFs
Photon Et (GeV)
25 30 35 40 45 50 55 60
Photon + b cross-section (pb)
0 10 20 30 40 50 60
CDF Run 2 Preliminary L = 66.7 pb-1
∫
LO (Pythia CTEQ5L)
(a) Photon+charm
Photon Et (GeV)
25 30 35 40 45 50 55 60
Photon + c cross-section (pb)
0 50 100 150 200 250 300 350
CDF Run 2 Preliminary L = 66.7 pb-1
∫
LO (Pythia CTEQ5L)
(b) Photon+bottom
Fig. 3.2.21: Transverse energy distribution for photons in events with a tagged heavy quark. The data is compared to PYTHIA.
The dominant systematic errors are the jet energy scale and the tagging efficiency; both of them are expected to decrease with luminosity, albeit not as quick as the statistical error; it is therefore likely that this measurement will start to be systematics dominated. Moreover, while the statistical errors in the various bins are uncorrelated, the effect of a change in PDF’s is likely to be a simultaneous shift of all bins in the same direction, so the biggest obstacle to PDF’s determination will be global effects like enregy scale,b-tagging efficiency and luminosity. Although they can be certainly be controlled with a precision at least a factor of 2-3 better than the present analysis, from the numbers in the table it is not likely that their precision can be better than the effect of varying the PDF’s within present limits, indicated as the last source of systematics. This measurement will probably not allow a direct determination of the PDF’s, however it will provade an extremely valid cross-check of the latters, that so far have only been indirectly derived from the gluon distribution. Another experimental approach being pursued by CDF on this measurement is the use of a dataset with a lower threshold on the photon at trigger level (12 GeV), but the requirement for a track with impact parameter measured on-line. This study will allow adding more high-statistics low-ET bins to the measurement, however the question remains if the trigger efficiencies will be understood at a sufficient level to reach the precision envisaged to observe effects due to PDF’s.
Heavy quarks andZ bosons
The production of beauty and charm in association with aZ boson decaying into electrons and muons is presently measured in CDF for an integrated luminosity of about 340pb−1. We require two opposite-charge leptons to lie inside aZ mass window, and a jet tagged as an heavy quark, with the same b-tagging
ET range (GeV) 25-29 29-34 34-42 42-60 Tag Efficiency +1.7-1.3 +2.6-2.0 +0.9-0.7 +1.1-0.9
Photon Id ±0.2 ±0.1 <0.1 ±0.1
Jet correction 0.5 +0.5 +0.1 +0.1
Jet energy scale +3.3-1.4 +2.2-2.1 +0.5-0.3 +0.5-0.4
B jet correction ±0.2 ±0.3 ±0.1 ±0.1
CPR fake estimate +0.1 <0.1 < 0.1 < 0.1
trigger +2.5-1.7 < 0.1 < 0.1 < 0.1
luminosity +0.7 − 0.6 +1.1 − 1.0 +0.4 − 0.3 +0.5 − 0.4
PDF ±0.3 ±0.5 ±0.2 ±0.2
Statistical 11.2 17.2 6.2 7.9
Systematics +16.4-8.2 +12.3-10.1 +6.4-4.4 +5.0-4.1
Table 3.2.1: Sources of systematic errors compared to the statistical one for the b-photon channel with a luminosity of 67pb−1.
tagging as the previous analysis. The leptonicZ channel (without b-tagging) is used as a normalization channel, to account from trigger and detector effects directly from data. The separate contributions from beauty, charm and light quarks are extracted, similarly to the previous analysis, from a fit to the vertex mass of the tagged jet (figure 3.2.22, left). Theη distribution of the selected quarks is shown in figure 3.2.22, right. The preliminary measured cross sections and branching fractions have presently a statistical error of about 30%, and a systematic error about half this value.
/ GeV/c2
MSVtx
-5 -4 -3 -2 -1 0 1 2 3 4 5
Jets
0 5 10 15 20 25 30 35 40
Data Total MC c b
+ b jet. CDF RUN II Preliminary Z0
Fit of Mass at Secondary Vertex
=1.96 TeV, L ~ 335 pb-1
s
>20 GeV
jet
ET
|<1.5 ηjet
|
data=101 N
MC =99 N
± 12
light=30 N
± 19 =23 Nc
± 15 =46 Nb
(a) Invariant mass of the tracks composing the secondary vertex
ηjet
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Jets
0 2 4 6 8 10 12 14 16
Data Total MC c b
+ b jet. CDF RUN II Preliminary Z0
Tagged Jets
=1.96 TeV, L ~ 335 pb-1
s
>20 GeV
jet
ET
|<1.5 ηjet
|
(b)η distribution of the tagged jets
Fig. 3.2.22: Distributions forZb events
We can assume that systematic errors will end up being around 10%,and that statistical errors of the same order of magnitude will be obtained with data already available. A further improvement towards a total (statistics + systematics) error of the order of 10-15% could be envisaged for the final Tevatron
dataset.
Heavy quarks andW bosons
The signature of a b quark and aW boson is characteristic of single top production, a signal long sought after since Run I. The final state searched for is an electron or muon plus missing energy, compatible with a W bosons, plus at least one b-tagged hadronic jet. The latest CDF publication [50] uses a data sample corresponding to an integrated luminosity of 162 pb−1, and puts a 95% C.L. of 17.8 pb for the combined cross section of s- and t-channel. As expected, from theη distribution in figure 3.2.23, most of the observed data comes from QCDW + b/c production.
Fig. 3.2.23: η of the reconstructed top candidate times its charge. This variable allows discrimination between s- and t−
channel production.
Table 3.2.2 shows the main sources of systematic uncertainty on single top production. The PDF error is the cross section difference between the “standard” set used in the analysis (CTEQ5L) and the one leading to the largest variation (MRST72). Using this conservative method, differences can be relevant even with the present limited statistics. Moreover, since the PDF influence is different for the s- and for the t-channel, the rapidity distribution, shown above, can yield additional information with respect to the simple cross section measurement.
Source Syst. error (%)
Energy Scale +0.1-4.3
Initial State Radiation ±1.0 Final State Radiation ±2.6
Generator ±3
Top quark mass -4.4
Trigger, lepton ID, Lumi ±9.8
PDF ±3.8
Table 3.2.2: Sources of systematic errors for the single top search (W b measurement)
Inclusiveb cross section
This measurement requires the presence of a tagged hadronic jet in the event, collected with a series of prescaled triggers with cuts on rising values of the jet transverse energy. A vertex mass method is used to extract theb fraction, and corrections for the b-tagging efficiency and jet energy scale are applied. This measurement, performed on an integrated luminosity of 300 pb−1 covers a jetPT range between 38 and 400 GeV, where the cross section spans over six orders of magnitude. The resulting cross section is shwn in figure 3.2.24.
jet [GeV/c]
PT
50 100 150 200 250 300 350
[nb/(GeV/c)]T/dYdPσ2 d
10-7 10-6 10-5 10-4 10-3 10-2 10-1 1 10
Data
Pythia Tune A (CTEQ5L) Systematics
CDF Preliminary = 1.96 TeV, L~300 pb-1
s
=0.75
merge
=0.7, f MidPoint jets Rcone
|Y|<0.7
Fig. 3.2.24: Cross section for inclusiveb production for a luminosity of 300 pb−1, compared to PYTHIA Tune A predictions
The main systematics for this measurement are summarised in table 3.2.3; since systematics are individually computed for each PT bin, in the table only an indicative value for the low-PT and the high-PT ends of the spectrum are given.
Source Syst. low-PT (%) Syst. high-PT (%)
Energy Scale +10-8 +39-22
Energy resolution ±6 ±6
Unfolding ±5 ±15
b fraction +14-15 +47-50
b-tagging eff. ±7 ±7
Luminosity ±6 ±6
PDF ±7 ±20
Table 3.2.3: Sources of systematic errors for the inclusiveb cross section
We see that the jet energy scale and the calculation of the b fraction largely dominate the error, and they increase at high-PT, where statistics of the control samples is scarcer. More data can certainly improve these errors, possibly by a factor of 2 in the low-PT and intermediate region, and more in the high-energy region. A global fit of the PT spectrum, and of the angular distribution will e needed to extract most of the information about PDF’s.
Conclusions
We highlighted some of theb production measurements recently performed in CDF, and their sensitiv-ity to PDF’s measurements. With present measurements we are still far from observing effects due to uncertainties in PDF’s in CDF data. A lot of work will be needed to reduce the systematic uncertain-ties, especially those relative to the jet energy scale and theb-tagging efficiency and purity. For doing that, the largest possible control samples are needed, so these measurements will benefit from as much data as possible. Even if they will end up being limited by systematics, the only way to reduce this systematics will be to accumulate more statistics. In any case, even if it will turn out that none of these single measurements will alone be able to constraint present errors on PDF models, they will consti-tute a fundamental direct cross-check of the validity of these distributions, so far only derived by QCD calculations.
3.3 Issues of QCD Evolution and Mass Thresholds in Variable Flavor Schemes and their Impact