Vector Boson Scattering prospects for High-Luminosity LHC at CMS in the same sign WW final state

Abstract

A feasibility study is presented for Vector Boson Scattering measurements in the same-sign WW final state, that can be performed during the high luminosity phase of the LHC. Particular emphasis is given to the expected performances of the detector in different upgrade scenarios, which are compared to each other and to the results attained with the current, aged technology. Presented at LP2015 XXVII International Symposium on Lepton Photon Interactions at High Energies Vector Boson Scattering prospects for High-Luminosity LHC at CMS in the same sign WW final state Raffaele Gerosa∗† INFN and University of Milano Bicocca E-mail: Raffaele.Gerosa@mib.infn.it A feasibility study is presented for Vector Boson Scattering (VBS) measurements in the samesign WW final state in view of the high luminosity phase of the LHC (HL-LHC). Particular emphasis is given to the expected performances of the CMS upgraded detector, which is compared to the results attained with the current aged and non-aged technology. Expected sensitivity to the longitudinal electroweak scattering, the non-unitarized scenario, in which the Higgs boson does not regularize the scattering cross section, and to anomalies in the quartic gauge couplings have been set through dedicated simulation studies. International Symposium on Lepton Photon Interactions at High Energies 17-22 August 2015 University of Ljubljana, Slovenia ∗Speaker. †On behalf of the CMS Collaboration c © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). http://pos.sissa.it/ Vector Boson Scattering in the same sign WW final state at CMS Raffaele Gerosa Precision measurements in the electroweak symmetry breaking (EWSB) sector represent important goals in the context of the high luminosity LHC program. Vector Boson Scattering (VBS) and quartic gauge couplings (QGC) are key aspects of the Standard Model (SM), which remain mainly unexplored after the first run of the LHC collisions. In fact, so far only an evidence for the electroweak WW production has been reported by both ATLAS and CMS experiments [1, 2]. The observed Higgs boson provides a candidate for a simplified EWSB mechanism, where vector bosons acquire mass and a longitudinal degree of freedom through their couplings to the Higgs field. These interactions allow also the unitarization of the scattering amplitudes of longitudinally polarized vector bosons (VLVL → VLVL), preventing the final result to be divergent at high energies [3]. If the SM is just an effective low energy theory, the VBS unitarization will be only partially operated by the Higgs boson and the complete regularization would happen via new physics, which intervenes at a large energy scale. In a large data sample, VBS may be used to determine that scale and possibly resolve different models that could be nearly degenerate at the Higgs pole, while showing different behaviors at higher energies. This poster presents studies of the CMS experiment potential at the HL-LHC inside this context [4]. The VBS signal exhibits when two vector bosons, emitted by a pair of incoming quarks, interact with each other. The two quarks get deflected from the beam direction and originate strong hadronic jets in the detector, called tag-jets, which clearly identify a scattering event. In this way, the HLLHC becomes a vector boson collider, providing access to purely electroweak processes, O(α6 ew) at Leading Order (LO), in a high energy regime. Any new resonances in the VBS invariant mass spectrum or a deviation of data from the SM expectations would be a ground-breaking result, a direct evidence of the new physics scale. The small cross section of the electroweak process, O(α6 ew), and the large contamination due to the production of vector boson pairs in association with two hadronic jets via strong interactions, O(α4 ewα s ), make this analysis very challenging. Moreover, in the harsh experimental conditions of the HL-LHC, an average of about 140 soft QCD interactions (pileup) will overlap with each other during the same beam collision. In this high-occupancy environment, background contamination also arises when hadronic jets are misidentified as leptons ( j-` fake rate), or jets from pileup interactions mimic the properties of VBS ones. For these reasons, the designed upgrade of the CMS detector (Phase-II) presents a new tracking system, extended up to |η |=4, and a new radiation hard and highly granular forward calorimeter, which are beneficial in reducing the j-` fake rate as well as the background produced by pileup events. In this study, only fully leptonic decays of the vector bosons are considered, to select topologies with lower backgrounds at hadron colliders, avoiding ambiguities due to the separation of the tag jets from the vector boson decay products. Projections are presented for same-sign WW scattering (pp→W±W± j j→ 2`±2ν j j). The sensitivity to the same-sign WW electroweak scattering is determined through the measurement of its inclusive and longitudinal cross sections. The sensitivity to a non-unitarized scenario, in which the Higgs boson does not play any role in the VBS, is studied as an indicator for the determination of new strongly-coupled sectors in the theory. Finally, new physics resulting in enhanced VBS cross section at high energy could be due to high-mass scalar resonances or new massive vector bosons, such as those predicted in unified theories. Such additional states would alter the strong interference between the VBS diagrams, leading to strong enhancements at high energy. These effects are described through the effective field theory (EFT) formalism, where dimension-six op-