Measurement of WZ and ZZ production in pp collisions at 8 TeV in final states with b-tagged jets with the CMS experiment

In this note we present a measurement of the VZ (V=W,Z) production cross section in proton-proton collisions at $\sqrt{s}=$8 TeV in the VZ$\rightarrow$V$b\bar{b}$ decay mode with V$=$Z$\rightarrow (\nu\bar{\nu}$,$\ell\ell$), V$=$W$\rightarrow \ell{\nu}$, ($\ell=e,\mu$). The results are based on data corresponding to an integrated luminosity of 18.9 fb$^{-1}$ collected with the CMS experiment. The process is observed for the first time in this particular final state with a significance exceeding six standard deviations ($\sigma$). The measured cross sections are consistent with the predictions of NLO calculations.Comment: 4 pages, 8 figures, proceeding for the Moriond 2014 EW sessio

A Novel Technique to Reconstruct the Z mass in WZ/ZZ Events with Lepton(s), Missing Transverse Energy and Three Jets at CDFII

Observing WZ/ZZ production at the Tevatron in a final state with a lepton, missing transverse energy and jets is extremely difficult because of the low signal rate and the huge background. In an attempt to increase the acceptance we study the sample where three high-energy jets are reconstructed, where about 1/3 of the diboson signal events are expected to end. Rather than choosing the two E$_{T}$-leading jets to detect a Z signal, we make use of the information carried by all jets. To qualify the potential of our method, we estimate the probability of observing an inclusive diboson signal at the three standard deviations level (P$_{3\sigma}$) to be about four times larger than when using the two leading jets only. Aiming at applying the method to the search for the exclusive WZ/ZZ$\rightarrow \ell \nu q\bar{q}$ channel in the three jets sample, we analyzed separately the sample with at least one $b$-tagged jet and the sample with no tags. In WZ/ZZ$\rightarrow \ell \nu b\bar{b}$ search, we observe a modest improvement in sensitivity over the option of building the Z-mass from the two leading jets in E$_T$ . Studies for improving the method further are on-going.Comment: Presented at the 2011 Hadron Collider Physics symposium (HCP-2011), Paris, France, November 14-18 2011, 3 pages, 8 figur

A Novel Technique to Reconstruct the Mass of the Associated Z boson Decaying into Jets in W+Z and Z+Z Events with Lepton(s), Missing Transverse Energy and Three Jets.

The study of associated WZ boson production with a lepton and a neutrino signaling the W, and a bb-pair in the final state is important since the event topology of this process is the same as expected for WH associated production of a W and the Standard Model light-Higgs boson (M_{H} lnubb whose rate can be accurately predicted, allows to calibrate and optimize many of techniques used in the SM Higgs search and provides a ``standard candle'' for that crucially important search. In addition, WZ associated production generates a significant background for low mass Higgs Boson searches with H decaying into a bb pair. At the Tevatron, the process WH-> W bb has an expected cross section about five times lower than WZ->Wbb for m_H~ 120 GeV/c2. Therefore, observing that process would be a benchmark for the even more difficult search for the light Higgs in the WH->Wbb process. Observing associated WZ production at the Tevatron in the channel WZ->lnubb is extremely difficult for two main reasons. The event rate is extremely low. A WZ production cross section of ~3.22 pb together with a Z->bb branching ratio of ~15% provides about 50 fb in the WZ->lnubb channel. With a trigger and kinematical selection efficiency of the order of a few %, one expects a handful of events per fb^-1 of integrated luminosity. This statement remains valid even if the few ZZ events with leptonic decay of one Z are included in the acceptance. A standard kinematical cut requests exactly two high energy jets (i.e. E_{T}> 20 GeV) in the candidate sample. Simulations show that if a third energetic jet is allowed the signal acceptance is increased by about 1/3. Therefore, it would be very important to be able to extract a Z->bb signal also in events with more than two high energy jets. A second difficulty is that the signal to background ratio is very poor, due primarily to the contribution of associated production of $W$ and incoherent jets. Optimal dijet mass resolution is of utmost importance for discriminating this background, since a fit to the invariant mass distribution of the two jets, associated to the hadronic decay of $Z$, is used to disentangle the diboson signal from the backgrounds in the candidate data sample. In this thesis, we present a search for WZ/ZZ in events with a lepton(s), missing transverse energy and jets. Besides looking at the sample where two exclusive jets are found, we investigate the sample with 3 jets where about the 33% of the signal events lie. In WZ events, additional jets may be initiated by gluon(s) radiated by the interacting partons (initial state radiation, ISR) or by the Z-decay products (final state radiation, FSR). FSR jets should legitimately be included in the reconstructed Z-mass. However, the presence of either ISR or FSR jets in a 3-jets events confuses the choice of the jet system to be attributed to Z decay. In this sample the invariant mass of the two E_T-leading jets would normally be chosen to reconstruct the Z boson. To improve both the mass resolution and the sensitivity of the search we describe an alternative procedure to reconstruct the Z-invariant mass. Improving the resolution in such a sample means choosing the correct jet combination for building the Z mass. My thesis work has been to investigate at generator level a sample of simulated CDF WZ events for finding a means to determine the origin of the extra jet and the right jet combination to be chosen for the best reconstruction of the Z mass. This is attempted for the first time in CDF. Four different Neural Networks (NNs) have been trained: NN$_{12}$, NN$_{13}$, NN$_{23}$ and NN$_{123}$. These NNs should make us to be able to decide event by event which among of the 4 four possible different combinations can be used for building the $Z$-mass in the three jets sample. If one jet is due to ISR, we expect one of the MJ1J2, MJ1J3, MJ2J3 combinations to be correct, while if one jet is due to FSR the choice should be MJ1J2J3. NNs combine kinematical information and some tools developed by CDF Collaboration for distinguishing gluon-like and b-like jets from light-flavored jets. Based on the response of the four NNs, we determine the most likely jet combination for building the Z mass in each event. The method allow to use a different combination from J1J2 in about 50% of cases. To qualify the potential of the method we have studied an experimental data sample accepting events with a leptonically decaying W and 3 large transverse momentum jets, as in the studies of the simulated WZ sample. The selection cuts accept jets of all flavors (pretag sample), and all diboson events including WW besides WZ, ZZ may pass the cuts. We estimate the probability at three standard deviations level to extract an inclusive diboson signal in the 3-jets sample alone (P$3\sigma$). After our procedure for building the Z mass is applied, P$3\sigma$ is about 4 times greater than when building the Z mass ``by default'' with the two E_{T} leading jets. The next step would be to discriminate against the WW contribution. A straight ``Higgs like'' approach would be to require b-jets in the events. When one or more jets are required to be b-like (the \emph{tag} sample) our technique, if applied stand-alone, provides only a modest improvement in sensitivity over the option of building the Z-mass from J1J2. Studies for improving the method further are on-going. However, already now our technique allows including the three jets sample in the WZ/ZZ analyses in order to increase acceptance and sensitivity in the search for the hadronically decaying Z-boson

Search for the Higgs boson in the bbˉ\mathrm{b\bar{b}} decay channel using the CMS detector

A search for the standard model Higgs boson in the \bbbar decay channel has been carried out with the CMS detector at the LHC collider. The searched production modes are the associated VH production, the VBF production and the production in association with top quark pairs (ttH). The analyses are based on pp collision data collected at centre-of-mass energies of 7 and 8 TeV, corresponding to integrated luminosities of 5 fb$^{-1}$ and 20 fb$^{-1}$, respectively. The strategy and results of the searches are reported.Comment: 7 pages, 7 figures, to appear in the proceedings of ICHEP 201

Prospects for a measurement of the WW boson mass in the all-jets final state at hadron colliders

Precise measurements of the mass of the $W$ boson are important to test the overall consistency of the Standard Model of particle physics. The current best measurements of the $W$ boson mass come from single production measurements at hadron colliders in its decay mode to a lepton (electron or muon) and a neutrino and pair production of $W$ bosons at lepton colliders, where also the decay mode of the $W$ boson to hadrons has been considered. In this study, prospects for a measurement of the $W$ boson mass in the all-jet final state at hadron colliders are presented. The feasibility of this measurement takes advantage of numerous recent developments in the field of jet substructure. Compared to other methods for measuring the $W$ mass, a measurement in the all-jets final state would be complementary in methodology and have systematic uncertainties orthogonal to previous measurements. We have estimated the main experimental and theoretical uncertainties affecting a measurement in the all-jet final state. With new trigger strategies, a statistical uncertainty for the measurement of the mass difference between the $Z$ and $W$ bosons of 30 MeV could be reached with HL-LHC data corresponding to 3000 fb$^{-1}$ of integrated luminosity. However, in order to reach that precision, the current understanding of non-perturbative contributions to the invariant mass of $W\to q\bar{q}'$ and $Z\to b\bar{b}$ jets will need to be refined. Similar strategies will also allow the reach for generic boosted resonances searches in hadronic channels to be extended.Comment: 23 pages, 10 figures, 6 tables, Fig.10 update

The alignment of the C3 Accelerator Structures with the Rasnik alignment system

The Rasnik 3-point alignment system, now widely applied in particle physics experiments and in the instrumentation of gravitational wave experiments, can be used as N-point alignment system by daisy chain N individual 3-point systems. The conceptual implementation of Rasnik chains in C3 is presented. The proper operation of a laser diode and a CMOS image sensor in liquid nitrogen has been verified. Next plans for testing a small but complete system, immersed in liquid nitrogen, are presented

Jet substructure at the Large Hadron Collider

Jet substructure has emerged to play a central role at the Large Hadron Collider, where it has provided numerous innovative ways to search for new physics and to probe the standard model, particularly in extreme regions of phase space. This review focuses on the development and use of state-of-the-art jet substructure techniques by the ATLAS and CMS experiments