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Higgs Searches in Run 2 at the Tevatron


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Higgs Searches in Run 2 at the Tevatron

Juan A. Valls

Rutgers, The State University of New Jersey

Physics Department, P.O. Box 849

Piscataway, NJ 08855

Abstract
In Run II at the Tevatron, the upgraded CDF and D0 experiments will have greatly improved sensitivity in the search for the Higgs bosons of the Standard Model and minimal supersymmetry. In the past year the Higgs Working Group of the Tevatron Run 2 SUSY/Higgs Workshop has estimated the discovery and exclusion reach for the Higgs, combining all possible search channels and utilizing all the upgraded features of both detectors. The results give strong motivation to continue the next run of the Tevatron into Run 3, with an eventual goal of up to 20 fb-1 or more in integrated luminosity delivered per experiment.




  1. Introduction

One of the most important goals of present and future high energy colliders is to reveal the mechanism responsible for electroweak symmetry breaking. In the Standard Model (SM) of electroweak interactions the Higgs mechanism introduces spontaneous symmetry breaking by the introduction of a scalar field doublet. This leaves a single observable scalar particle, the Higgs boson, with unknown mass but fixed couplings to other particles. Theoretical considerations suggest that other viable framework incorporating light elementary scalar fields requires “low energy” supersymmetry, where the scale of supersymmetry breaking lies no higher than O(1 TeV). The Higgs sector of the Minimal Supersymmetric extension of the SM (MSSM) is of particular interest because it predicts the existence of a light CP-even neutral Higgs boson with a mass below about 130 GeV/c2.
Precision electroweak experiments suggest that the Higgs boson could be light, with a central value in the range 90-100 GeV/c2 [1]. Direct searches at LEP rule out a SM Higgs with a mass below 95 GeV/c2 at 95% CL. The four LEP 2 experiments will eventually discover the Higgs or increase this limit to about 106-109 GeV/c2. This makes the mass range from 110-200 GeV/c2 specially interesting for the Tevatron. The goal of the Higgs Group experimental studies [2] is to estimate the discovery reach for the SM and MSSM Higgs bosons in Run 2 and beyond at the Tevatron. Results will ultimately be expressed in terms of the integrated luminosity required to either exclude the Higgs with 95% confidence if it does not exist, or discover it with some statistical significance if it does exist at some mass. The results obtained at the Workshop are based on different assumptions on identification efficiencies, b-tagging efficiencies, mass resolutions, background rates, and Run 2 detector simulations. Here we will highlight only basic standard signal selections.


  1. Standard Model Higgs Search

At Tevatron the most sensitive Higgs production mechanism is associated production with a vector boson (W or Z) with cross sections in the range 0.1-1.0 pb. Single Higgs production, with highest rates, becomes important only for non hadronic Higgs decays (HWW) since for Hbb the background from dijet events is too large. The searches for the SM Higgs divide into the Higgs mass regions below and above 135 GeV/c2, the mass at which the dominant Higgs decay modes change over from bb to WW.
2.1) Low Mass Region: mH = 90-130 GeV/c2

At lower masses, the decay mode of the accompanying W or Z in associated production determines the final state. The largest rate is to quark pairs, giving a qqbb final state although with limites sensitivity [3]. The next larger is the case where Wllbb channel), followed by the cases where one has Zl+l- and Zl+l-bb and bb channels). The selection for WH events where Wlrelies on events selected with a high pT lepton (e or ) trigger, mising transverse energy, and two b-tagged jets. Dominant backgrounds arise from tt, Wbb, single top and WZ events. ZH events with Zare quite distinct. The selection requires two 15 GeV- pT b-tagged jets and large missing transverse energy. Main backgrounds here are Zbb and ZZ events. In the mode where we have ZH with Zl+l-, the events are selected from a low pT dilepton smple. One demands the presence of ee and  pairs with an invariant mass consistent with that of the Z, recoiling against two b-tagged jets. Backgrounds here are dominated by real Z’s produced in conjunction with bb pairs or W decaying hadronically.


2.2) High Mass Region: mH = 120-190 GeV/c2

For SM Higgs masses above about 135 GeV/c2 the decay mode HWW dominates, and provides a means to potentially observe the Higgs. The main problem to overcome here is the roughly 10 pb cross section for vector boson pair production with signal rates 10-100 times smaller. Three channels have been shown to be of potential use in this mass regime: like-sign lepton pairs with jets, dileptons with missing transverse energy, and trilepton final states. The production of a SM Higgs decaying to W in association with a vector boson gives rise to WWW and ZWW final states with distinct low background signatures like two leptons of the same charge, two jets and missing transverse energy. Main SM background sources are tt, WZ+jets and fake leptons. At large masses, the most sensitive signature is two leptons and missing transverse energy from single Higgs production and subsequent decay HWWl+l-. The main challenge is to overcome the very large background from WW,WZ, ZZ and tt production. Finally, at large masses, with WWW and ZWW final states, one can also consider searching for trilepton events. This signature leads to low experimental backgrounds by removing events with lepton pair masses consistent with that of the Z. Additional “golden” combinations with same-sign leptons of the same type can also be used.




  1. Combined Channels SM Higgs Results

No single channel from the above described can be considered a “golden” channel. To maximize the sensitivity of the Higgs search it is necessary to combine the results of all channels and both experiments. The required integrated luminosity for either discovery or exclusion at the 95% CL increases exponentially up to Higgs masses of 140 GeV/c2, beyond which the high mass channels play a dominant role. In Run 1 (2 fb-1) the 95% CL limits will barely extend the expected LEP 2 limits, but with 10 fb-1 in Run 3, the SM Higgs can be excluded up to 190 GeV/c2 if it does not exist in that mass range. In Run 3, if a SM Higgs exists with a mass less than 180 GeV/c2 the combined sesitivity of CDF and D0 will yield an observation at the 3 level with 20 fb-1. However, a 5 discovery does not appear possible below just under 120 GeV/c2.


  1. MSSM Higgs Search

In the MSSM one has 5 physical Higgs states: two neutral scalars (h and H), one neutral pseudoscalar (A), and two charged Higgses (H).The masses and couplings of these two bosons are governed by two parameters usually taken as mA and tan. For much of the MSSM parameter space the h and H behave like the SM Higgs, and the results of the SM Higgs search applies directly. With 5 fb-1 one can exclude most of the parameter space in the plane mA-tan independent of other parameter assumptions affecting the stop quark mixing. To discover the MSSM Higgs in most of the space at 5 significance requires about 20 fb-1. However, there are still holes in this plane which can be in some cases quite large.

In addition, in the MSSM, the basic bb =h,H,A couplings can be greatly enhanced with respect to the SM because of the large Higgs bottom Yukawa couplings. This leads to enhanced Higgs production in conjuction to bb pairs, with relatively clean bbbb final state signatures. The integrated luminosity needed to rule out or discover the MSSM Higgs in this channel has been estimated by both the CDF and D0 experiments with different simulation and analysis assumptions. The basic selection for the case of the CDF analysis [4] starts by requiring a multijet trigger demanding four jets with a minimum of 15 GeV transverse energy and a total transverse energy in the event equal or above 125 GeV. At least three jets are required to be tagged as b jets. By far, the largest background comes from QCD bbjj production estimated from Monte Carlo and scaled by a factor determined with Run 1 data. This search represents the main mode for discovering or ruling out the MSSM Higgs at large tan (>20).




  1. Conclusions

The Higgs Working Group at the Tevatron Run 2 SUSY/Higgs Workshop has studied the discovery reach for the SM and MSSM Higgs boson in eight separate channels. Combining the results of these channels, and combining the data from both experiments, a SM Higgs can be excluded in Run 2 at the 95% CL up to 120 GeV/c2 mass. Its discovery at the 3-5 level would require10-30 fb-1 per experiment for, up to a 190 GeV/c2 mass. These results can also be interpreted in the context of the MSSM. A minimal SUSY Higgs can be excluded at the 95% CL over the entire parameter space with 10 fb-1. With 20 fb-1 a discovery at the 5 level would require 20 fb-1.
References
[1] LEP Electroweak Working Group. Winter 99 summaries. http://www.cern.ch/LEPEWWG/

[2] Physics at Run 2 Supersymmetry/Higgs Workshop. Higgs Working Group. Final report in preparation. http://futh37.fnal.gov/susy.html.



[3] F. Abe et al. Phys. Rev. Lett. 81, 5748 (1998)

[4] J. A. Valls, CDF Run 2 Discovery Reach for Neutral MSSM Higgs Bosons via ppbbbbbb. Fermilab-CONF-99-164-E, Nov. 1998.


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