The HCC130 testing and irradiation has been able to determine that HCC130 chip is very stable and does not experience a large current increase at low TID. A few design errors were uncovered such as the lack of protection in the interlock mechanism causing the interlock to be triggered early if the channel the interlock is placed on is read back and the max count of the AM is not properly kept at 1023 but instead rolls over.
While the HCC130 is being tested, the HCCstar and the AMAC are being designed. The HCCstar’s AM is much simpler and has a more complex data read and receive architecture than the HCC130. The HCC130’s AM capabilities are transferred to the AMAC. Future studies would
Figure 4.34: HCC130 current as a function of elapsed hours since the beginning of data taking (left), and of TID (right). The plots represent average current with the fluctuations due to the reading of the AM and daily reset of the HCC130.
Figure 4.35: HCC130 Bandgap AM counts as a function of elapsed hours since the beginning of data taking (left), and of TID (right). Before irradiation, the bandgap AM count is stable. Irradiation causes the AM counts to jump 25 counts and oscillates around that count value.
include conducting the same kinds of tests performed on the HCC130 but on these new chips since those chips are the ones that will be in the final ITk Strip detector.
Figure 4.36: Calibrating the AM during irradiation by varying the input voltage and reading out the AM output. Resulting ramps are on the bottom, with the resulting slopes (top left), and y-intercepts (top right).
Figure 4.37: Number of error reads for each of the control registers 0-3 (left) and the sum of SEUs that occur in registers 0-17 (right) during the HCC130 irradiation.
W±Z
cross-section measurement at√
s
= 13
TeV
Diboson cross section measurements provide important tests of the electroweak (EWK) sector of the Standard Model (SM) by measuring precisely the triple and quartic gauge couplings. Any deviation in the measurement of the strength of these couplings would provide evidence for physics beyond the SM.
Diboson refers to W Z, W W, and ZZ processes. The production of W Z is a good probe for diboson processes. Figure 5.1 shows the leading order diagrams for theW Z production in proton- proton collisions. The s-channel diagram has a triple electroweak gauge boson interaction gauge, which is a feature of the non-abelian structure of the group describing EWK interactions, but is also sensitive to new physics. Deviations in this coupling would lead to an enhancement in the production cross section. Limits on anomalous triple gauge couplings, as well as on anomalous quartic gauge couplings, which can be probed via vector boson scattering production, have been placed in Run 1
Z
W
±W
±q
¯
q
′Z
q
W±
q
¯
q′
Z
q′
W±
q
¯
q′
Figure 5.1: Leading-order diagrams forW±Z production inppcollisions.
by ATLAS [77, 78] and CMS [79, 80, 81].
TheW W process also receives contributions from leading order triple gauge coupling; however, theW Z process is an experimentally cleaner signature with fewer backgrounds. The W W process has large background contribution from t¯tproduced with associated jets. In order to reduce this background, a jet veto needs to be imposed, leading to larger jet systematic uncertainties. Moreover, the final leptonic decay ofW W is`ν`νwhich contains two invisible particles, while theW Z process contains only one invisible particle if the W boson decays leptonically, making the W W process kinematically more challenging to reconstruct thanW Z. The cross section times branching ratio is larger forW Z than forZZ production process.
As discussed in Section 2.1.4, final states with either the W or Z bosons decaying to hadrons have the largest branching ratio; however, they also have large backgrounds from many processes such as multijet,t¯t,Z+jet, andW+jet production. The final state where both theW andZbosons decay leptonically, also known as the three lepton channel, is therefore used to measure theW Z cross section. Z decays to neutrinos are not considered because they are not detected in the ATLAS detector. The three lepton channel includes decays to electrons and muons. Decays to taus are not considered directly. Taus have a 65% branching ratio to hadrons. The remaining 35% branching ratio is to leptons (electrons and muons). Leptonicτ decays have final states of the form µ¯νµντ or eν¯eντ. These states are indistinguishable from promptly produced electrons or muons, except for the additional missing energy from the tau neutrino. Thus, events with leptonically decaying taus contribute to the signal regions of the four channels used in the measurement region. This contribution is estimated using simulation and accounted for using correction factors during the calculation of the cross section.
The cross section of the W Z production decaying to three leptons was measured in proton- antiprotons collisions and published by the CDF and D0 collaborations [77, 78], as well as in proton- proton collisions at the LHC by ATLAS at √s= 7,8 TeV [82, 83], and by CMS at √s= 7,8,13 TeV [79, 80, 81].
The analysis presented in this chapter measures the fiducial cross section of theW Z production decaying to three leptons in theeee,eeµ, µµe,µµµ decay channels using 3.2 fb−1 of ATLAS data collected at a center-of-mass energy of 13 TeV. The four decay channels are combined using a χ2 minimization technique. The measurement is also extrapolated to the total phase space to determine the inclusive cross section. The paper on which these results are based also present the cross section as a function of jet multiplicity [2].
5.1
Cross section methodology
The cross section is the probability of an event occurring. The number of events produced depend on the cross section and the luminosity. Thus, the total number ofW Z events produced is determined using the equation
NW Z =L ·σ (5.1)
NW Zis determined by the total number of data events observed,N, minus the number of background events,B. Detector effects such as lepton identification, trigger, and others are accounted for with a correction factor,C, and the finite acceptance of events is accounted with for with an acceptance factor,A. Combining all this, the total cross section becomes,
σ= N−B
L ·C·A (5.2)
5.1.1
Fiducial cross section
A fiducial cross section measurement (differential or inclusive) is calculated such that the measure- ment can be directly compared with theory, with little dependence on the underlying model (“model independent” cross section measurement). The phase space selection for this measurement can be either geometric or kinematic. The geometric selections ensures that events will be within the de- tector while a kinematic selection reduces background in the measurement region that are difficult to model. The fiducial phase space selection can be further subdivided as a function of one or more kinematic properties. This cross section measurement is known as a differential cross section.
After events are selected within the fiducial phase space, background events are subtracted. The remaining events must be corrected to account for detector-level effects such as lepton identification, trigger efficiency, or resolution on kinematic variables, a process known as unfolding. Unfolding is done with as few assumptions on the model as possible. For an inclusive cross section, unfolding is a single number, called a correction factor, CW Z. The procedure is more complicated for a differential cross section because of event migration between bins.
The fiducial cross section is calculated as
σWfid.±Z→`0ν``=
Ndata−Nbkg
L ·CW Z
. (5.3)
The correction factor is the ratio of the number of events in the fiducial phase space using recon- structed events over the number of events derived from the Monte Carlo simulation in the fiducial
phase space, so that
CW Z =
Nsignal reco
Nfiducialsignal . (5.4)
This will be discussed further in section 5.7. To compare with theory, events generated using a Monte Carlo generator have the fiducial selection applied. The resulting cross section can be compared with the experimentally measured cross section.
5.1.2
Total cross section
The fiducial cross section can be extrapolated into a cross section in the total phase space. This extrapolation takes into account the branching ratios of W andZ bosons to three leptons as well as an acceptance factor, AW Z. This acceptance factor is a truth-level factor that extrapolates from the fiducial phase to the total phase space. The factor is given by
AW Z =
Nfiducialsignal
Ntotalsignal . (5.5)
Determining this factor will be discussed further in section 5.7. The total cross section is calculated as equation (5.6): σW Ztot = σ fid. W±Z→`0ν`` BRW→`ν·BRZ→``·AW Z (5.6)