Dibosons from CMS

It is here presented the diboson production cross section measured by the CMS collaboration in $pp$ collisions data at $\sqrt{s}$=7 TeV. $W \gamma$ and $Z \gamma$ results from 2010 analyses (36 pb$^{-1}$) are presented together with 2011 first measurements of $WW$, $WZ$ and $ZZ$ final states obtained using 1.1 fb$^{-1}$. Results obtained with 2010 data are also interpreted in term of anomalous triple gauge couplings.


Introduction
The diboson production is a direct probe of the Standard Model. Its high sensitivity to the self-interaction between gauge bosons is a direct consequence of the non-abelian S U(2) × U(1) gauge symmetry of the SM and is fully fixed in the SM by the gauge structure of the Lagrangian. Any deviation of the SM couplings is an indication of new physics, manifested as an increased production cross section for instance. The measurement of triple gauge boson couplings (TGC) is also an important test of the SM as a useful step to establish major backgrounds to the electroweak searches. Moreover, a number of extensions of the SM can manifest themselves in processes with multiple bosons in the final state and a measurement of TGCs can be sensitive to new phenomena at high energies, that would require more energy or luminosity to be directly observed. Within the SM, using the effective Lagrangian approach, g V 1 , k V 1 , λ V and h V 3 , h V 4 operators are used to describe the charged [5] and neutral [6] couplings respectively, which are Lorentz and S U(2) × U(1) invariant and conserve C and P separately. In the following the 95% CL intervals for anomalous TGCs are presented, using the HISZ parametrization. No form-factor is used in the interpretation of the results in order not to make any assumption on the energy dependency of new physics that would ensure partial-wave unitarity.
A complete description of the detector and informations about objects reconstruction can be found in [4]. The analyses here presented use data from pp collisions at 7 TeV registered by the CMS detector in 2010 (36 pb −1 ) and 2011 (1.1 fb −1 ), focusing on the fully leptonic final states reconstructed with high efficiency over a very wide acceptance. Results are presented in the following, with a brief description of the diboson events selection. a e-mail: arabella.martelli@cern.ch 2 Wγ and Zγ production cross section Wγ and Zγ diboson processes are studied with analyses sharing similar strategy and techniques. W bosons are reconstructed if only one isolated lepton with p T >20 GeV/c is found together with a missing transverse energy above 25 GeV/c, while the Z candidates are selected looking for two isolated leptons with p T >20 GeV/c and a dilepton invariant mass above 50 GeV/c 2 . For both analyses, only photons with p T >10 GeV/c are considered. Moreover an angular separation in terms of ∆R(γ, lepton) = (∆η) 2 + (∆φ) 2 > 0.7 is required. The main backgrounds are due to jets misidentified as photons and are estimated with data driven techniques. Both W+jets and Z+jets background contributions are estimated by measuring the probability for a jet to be identified as a photon candidate in a sample of multi-jet QCD events containing at least one high-quality jet candidate, and then folding this probability with the non-isolated photon candidates observed in the Wγ and Zγ samples. Since only electrons and muons final states are considered, the fraction of Wγ events decaying into taus are subtracted as a background, once estimated from the simulation (order 3%).
The number of events observed in data and estimated for the backgrounds in each leptonic final state is shown in Table 1 for the Wγ and in Table 2 for the Zγ analysis [1]. No event is observed with more than one photon candidate in the final state.  The main systematic uncertainty is due to the estimate of backgrounds from data and amounts to ∼6% for Wγ and to ∼10% for Zγ. Other significant sources of systematic uncertainties are from the PDF modeling and photon energy scale, contributing by ∼5% for all final states. The Wγ cross sections measured in each final state are weighted taking into account correlated uncertainties between the two results. The combined cross sections is This result agrees well with the NLO prediction [8] of 49.4 ± 3.8 pb. By following the same procedure, the corresponding cross section measured for the Zγ production is The theoretical NLO prediction [6] is 9.6 ± 0.4 pb, which is in agreement with the measured value.

WW production cross section
The signature looked for a WW event consists in exactly two well isolated leptons among electrons and muons only, with p T >20(10) GeV/c for the leading (trailing) lepton, together with a significant missing transverse energy (MET ) to account for the two neutrinos. For this purpose a particular variable is used, namely the missing transverse energy is projected along the direction of the closest lepton and its orthogonal component is used as discriminating variable in case the angle ∆φ(MET, lepton) is smaller than π/2. This "projected-MET " is particularly suitable to reject eventual Z → ττ decays, in case of boosted Z. It is required above 40 GeV for the same flavor W decays (ee, µµ) and above 20 GeV for the eµ channels. Some further cuts are required to reduce the contamination from background processes with jets, heavy hadrons and mis-reconstructed diboson channels. Events are rejected if containing jets with p T >30 GeV/c, a further veto is applied on "top-tagged" jets accounting also for soft muons from b-quark decays. To further reduce the Drell-Yan background, events with a dilepton e + e − , µ + µ − invariant mass within 15 GeV/c 2 of the Z are vetoed. Finally the angle in the transverse plane between the dilepton system and the most energetic jet with p T >15 GeV/c is required to be smaller than 165 degrees, in the ee/µµ final states, to cope with Z events where the boson recoils against a jet. The main backgrounds are estimated directly from data. The fake rate measured on jets enriched data samples allows to estimate W+jets and QCD multi-jet events. The remaining top background is estimated from data as well by using "top-tagged" events and applying the corresponding tagging efficiency, which is measured in a data control sample. The residual Z boson contribution to the e + e − and µ + µ − final states are estimated by normalizing the simulation to the observed number of events inside the Z mass window in data.
The number of expected signal and background events, for the processes controlled with the simulation and with data-driven techniques are reported in Table 3 [2]. Table 3. Number of events observed in data and expected event yield for signal and background, estimated from data and simulation for L dt=1.1 fb −1 , after applying the WW selection requirements.

Sample
Yield The spectrum of the projected-MET at the end of the event selection is shown in Figure 1  The major systematic uncertainty is from the background estimation from data (order 20%), while ∼ 8% accounts for the signal efficiency. The signal acceptance corresponding to the selection described is order 70% of the total phase space. By using the W → lν branching fraction, the WW production cross section is measured to be This measurement is consistent with the SM expectation of 43±2 pb at NLO [7] within one standard deviation.

WZ production cross section
In the WZ → l νl + l − channel, three isolated leptons are looked for considering electrons and muons only. Leptons candidates from Z decays are first selected with p T >20,10 GeV/c for electrons and p T >15,15 GeV/c for muons and the Z boson is chosen as the best invariant mass candidate within the range [60, 120] GeV/c 2 . Then a third lepton satisfying tight identification and isolation criteria and with p T >20 GeV/c is required as well as a significant missing transverse energy (above 30 GeV) associated to the neutrino so to select events containing also a W boson. Events with a second Z candidate reconstructed are rejected. The dilepton invariant mass distribution for events surviving the signal selection is presented in Figure 2 for simulation and data. The observables are the total number of events in these two samples. The Loose-cut sample contains events with the W candidates reconstructed from either true isolated leptons (N lep ) or from fake ones from misidentified jets (N jet ) or from leptons from heavy flavour decays. Hence the number of events in this sample can be expressed as:

Background Estimation
The number of events in the Tight-cut sample can be written as: where tight is the efficiency for true isolated leptons to pass the isolation cuts and P fake is the corresponding efficiency for fake leptons. We will obtain tight and P fake from independent Z+jets and tt are estimated from data with the "matrixmethod". This technique uses a tag-and-probe procedure to measure the lepton selection efficiency and the probability for a jet to fake a lepton, using enriched Z and Z+jets data samples. ZZ and Zγ are backgrounds in case of leptons not reconstructed or photon conversions and are controlled with the simulation. WZ events where bosons decay into tau lepton(s) are considered as background and the fraction of such events as estimated from the simulation (order 6%) is subtracted to the same number of events observed in data. The event counts observed in data are reported in Table 4 together with the estimated background and signal event yield [2]. The signal acceptance corresponding to the selection described is order 50% of the total phase space. Concerning the systematic uncertainty, major sources are from the lepton selection and the background control. In particular a 20% systematic uncertainty is assigned to ZZ and Zγ processes, which are estimated from the simulation being minor backgrounds, while a systematic uncertainty of ∼5% is assigned to the processes controlled from data. The cross sections measured channel by channel within the Z boson mass range [60, 120] GeV/c 2 are reported in Table 5. The inclusive WZ cross section is computed as a weighted mean taking into account the correlated systematic uncertainties and using the W and Z bosons branching ratios. It is measured to be consistent with the Standard Model prediction [7].

ZZ production cross section
The ZZ final state is reconstructed out of two pairs of same flavor, opposite charge, isolated leptons, using electrons, muons and taus. To select the first Z candidate electrons and muons only are considered, if having a p T >20(10) GeV/c for the leading(trailing) lepton and invariant mass within [60, 120] GeV/c 2 , while also taus are taken into account to look for the second Z. A p T >7(5) GeV/c is required for electrons (muons). Different criteria are used to select taus, namely p T >10 GeV/c for leptonic tau decays, p T >20 GeV/c for hadronic taus decays. Moreover, in presence of taus, it's the visible mass of the Z boson to be required in the range [30, 80] GeV/c 2 . Leptons are required to be isolated, the isolation being measured based on the combination of tracker, ECAL and HCAL measurements. Finally, a cut is applied on the lepton impact parameter for selected leptons. Leptons from misidentified jets or heavy hadrons decay are a main source of remaining backgrounds. Concerning final states with taus, also WZ can be a relevant background and it is estimated from simulations. Zbb in particular and tt are estimated from data, by means of a control region defined by reverting the cut on the leptons impact parameter. Z+ jets is controlled by measuring the rate of jets faking electrons, muons and taus respectively. For this purpose a data sample enriched in background is selected, by requiring a Z candidate as for the signal plus a pair of same flavor same sign leptons, without isolation criteria.
The spectrum of the four-lepton invariant mass at the end of the event selection is shown in Figure 3 for the final states with electrons and muons only.
In Table 6 the number of events observed in data, estimated for the backgrounds and as expected for the signal are reported [2].

Background estimate
The small number of observed events precludes a precise evaluation of the background, e.g. in a relevant signal-like mass window, only from the measurement of side-bands. We thus rely on other methods, based on experimental data, for the control of the background and the evaluation of the associated systematics. The typical procedure consists of choosing a wide background control region outside the signal phase space which gets populated by relaxing some cuts in the event selection, and verifying that the event rates change according to the expectation from Monte Carlo. The control region for any given background must be chosen carefully since any or both of the other two reducible backgrounds might rapidly become dominant if the event selection is relaxed, thus making the extrapolation to the signal phase space difficult.
For the measurement of the Zbb/cc and tt backgrounds, a four-lepton background control re-  The presented selection reduces the full signal phase space to order 60% for the e, µ and to ∼20% for tau channels respectively. Main sources of uncertainties are from the background control through data driven techniques and the selection of the leptons. As an example, a 3% systematic error is assigned to the e, µ reconstruction, while a 6% to taus. To include all final states in the cross section calculation a simultaneous constrained fit on the number of observed events in all decay channels was performed, taking into account the systematic uncertainties found, by means of a profile likelihood. The resulting ZZ production cross section for a pair of Z bosons in the mass range [60, 120] GeV/c 2 is measured to be

Limits on gauge couplings
Limits on triple gauge couplings were set using 36 pb −1 of data from 2010 pp collisions at 7 TeV. To measure such parameters, profile likelihood fits are performed on a relevant spectrum, taking the SM prediction as reference for comparison with the measurement. This procedure allows to quantify eventual anomalous coupling parameters, which would bring to an enhancement of the diboson production cross section, in particular at high boson transverse momentum, if introduced in the SM Lagrangian. The presented 95% C.L. intervals for the measured TGCs are obtained by varying one of the couplings, while fixing the remaining ones to the SM values.
Within the Wγ and Zγ analyses described in section 2, WWγ, ZZγ and Zγγ were measured, by looking at the spectrum of the photon transverse energy [1]. The results presented in Table 7 already allow for a good sensitivity to the neutral anomalous couplings. Also the WW analysis here presented was already performed in 2010 providing limits on the WWγ, WWZ couplings. For this purpose the discriminating spectrum used was the leading lepton p T [3]. Results are shown in Table 8. Table 8. 95% C.L. limits on one-dimensional fit results for anomalous TGC obtained within the WW analysis for L dt=36 pb −1 of 2010 data at 7 TeV.