Higgs Searches at the Tevatron
Kazuhiro Yamamoto
Department of Physics, Osaka City University
for the CDF collaboration

1Introduction

The standard model (SM) of the particle physics has been achieving quite successful outcomes interms of not only explanation of the already measured physics framework but also prediction ofthe variables to be measured. However, the key element of the SM, the higgs boson, has not been
discovered yet, which is indispensable for generating masses of particles and keeping the theory
renormalizable at the electroweak scale. Since it is also clear that the SM is not self-contained,that is, not enough to tell its framework without any given parameters, we expect the existenceof the physics beyond the SM. Among a number of physics models beyond the SM, the simplest
one is the minimal supersymmetric extension of the standard model (MSSM). Along with theabove theoretical guidance, searches for the SM and MSSM higgs bosons are being preformed
and/or being prepared with the worldwide collaborations. Here we present the recent results of
the higgs searches at the Tevatron, which is the only one facility where the higgs searches areundergoing at of this moment in the world.

2The Tevatron collider and the CDF detector√The Tevatron is a collider accelerator which provides pp̄ collisions at s = 1.96 TeV. As ofJanuary 2006, the typical peak luminosity and the weekly integrated luminosity is 1.5 ∼ 1.7×1032
cm−2 s−1 and 15 ∼ 20 pb−1 respectively. 1540 pb−1 has been delivered to the collision hall, 1230
pb−1 has been recorded to the data tapes, and about 360 pb−1 has been analyzed. The CDF
detector is a general-purpose detector for the collider experiment. The schematic view of theCDF detector is shown in Figure 1. Silicon strip detectors and a cylindrical drift chamber areplaced inside the solenoid magnet which provides the ﬁeld strength of 1.4 T. On the other hand,electro-magnetic and hadron calorimeters are located outside the magnet. Muon chambers areinstalled in the outermost region. The details on the CDF detector can be found in [1].

Central EM
Calorimeter

SolenoidCentral Muon

Central Hadron
Calorimeter
Forward Muon
Endplug Hadron
Calorimeter

Endplug EM
Calorimeter

TOF
Tracking Chamber
Silicon Detector

Figure 1: The schematic view of the CDF detector.

3SM higgs boson searches

The SM higgs boson is dominantly produced via the gluon fusion (gg → h) and the vector-boson
associated process (q q̄ (0) → W h/Zh) at the Tevatron energy. The cross sections of gg → h,q q̄ 0 → W h and q q̄ → Zh are 1.0 pb, 0.15 pb and 0.091 pb for the higgs mass (Mh ) of 120
GeV/c2 , and 0.43 pb, 0.050 pb and 0.033 pb for Mh = 160 GeV/c2 according to the NNLOcalculation [2]. The search channels are roughly grouped into two by means of the decay modesof which branching fraction depends on the higgs mass, namely h → bb̄ and h → W W . The
former one is promising for the light higgs (Mh < 130 GeV/c2 ) with the combination of thevector-boson associated production, while the latter one is suitable for the high mass higgs(Mh > 130 GeV/c2 ). The important thing is that we need at least one high-pT lepton fromW/Z and/or higgs itself for event triggering due to the quite large QCD background.
As the recent results from the CDF experiment, the following four higgs search channels arepresented here. Firstly, we describe the analysis of q q̄ 0 → W h → `νbb̄. The signature is high pTisolated lepton + missing ET ( > 20 GeV/c2 ) + two jets. We required at least one b-tagged jet.
The major components of the background are W + heavy ﬂavours and top quark production.
Since we did not see any signiﬁcant excess in the dijet invariant mass distribution, the upperlimits on the production cross section times branching ratio (σ(pp̄ → W h) × Br(h → bb̄)) wascalculated to be 3 ∼ 10 pb for the higgs mass range between 110 GeV/c2 ∼ 150 GeV/c2 at 95%
conﬁdence level (C.L.) with the integrated luminosity of 320 pb−1 [3]. Secondly we show theresult on q q̄ → Zh → ν ν̄bb̄. This is the most sensitive channel in the Tevatron Run I search
and also recent analyses at CDF. The signature of this channel is two jets and large missing
ET coming from two neutrinos in the ﬁnal state. We required jets to be at least one b-tagged,and missing ET to be larger than 70 GeV. We found the events passing the above cuts wereconsistent with the SM prediction which was dominated by QCD + mistag events and W/Zdiboson production. We set the upper limits on the production cross section times branchingratio (σ(pp̄ → Zh) × Br(h → bb̄)) to be 4 ∼ 6 pb for the higgs mass range between 90 GeV/c2∼ 130 GeV/c2 with 95% C.L. The integrated luminosity of 289 pb−1 was used for this channel.
Thirdly we investigate the high mass higgs via gg → h → W W . The advantage of this channel
is the large cross section of the higgs production through the gluon fusion. The signature is highpT opposite-sign dilepton from W + W − → `+ `− ν ν̄ and large missing ET . Also since the scalar
nature of the higgs boson results in the angular alignment between the ﬁnal state leptons unlike
those from diboson production which is the main component of the background, we utilized it
to search for excess in spectra of the dilepton invariant mass and dilepton azimuthal separation.
From the result of no excess against the SM prediction, the 95% C.L. upper limits on theproduction cross section times branching ratio (σ(gg → h) × Br(h → W W )) was obtained tobe 4 ∼ 9 pb for the higgs mass range from 110 GeV/c2 to 200 GeV/c2 using the integrated
luminosity of 360 pb−1 . The fourth channel is q q̄ 0 → W h → W W W . This process provides uswith a like-sign dilepton as a ﬁnal state resulting from W W W → `± ν`± νX. Since the like-sign
dilepton event is quite rate in the SM events, this should be a good signature to discriminate thenon-higgs SM events. Therefore, the background is dominated by events such as fake leptons andconversion electrons which are not able to be estimated by the simple Monte Carlo simulation.
After understanding eﬀects of the fake leptons and conversion electrons, no signiﬁcant excess
indicating higgs production was found in the 193.5 pb−1 data. We set the upper limits on theproduct of the production cross section and the branching ratio (σ(pp̄ → W h) × Br(h → W W ))to be around 10 pb at 95% C.L. for the higgs range from 90 GeV/c2 to 200 GeV/c2 . The
summary of the above results on the SM higgs searches are presented in Figure 2.
The SM higgs mass is also strongly correlated with masses of a W boson (MW ) and a top
quark (Mt ) via the radiative correction. Recently a new result of Mt was given by CDF andCross-Section × Br (pb)Tevatron Run II Preliminary– ––ZH→ννbb
WH→eνbb-1
D0: 382 pb-1 D0: 261 pb

H→WW(*)→lνlν
D0: 299-325 pb-1

WH→WWW
CDF: 194 pb-1

10– –ZH→ννbb-1
CDF: 289 pb

1–

WH→lνbb
CDF: 319 pb-1(*)

H→WW →lνlν
-1
CDF: 360 pb

WH→WWW
D0: 363-384 pb-1(*)

SM gg→H→WW–

10-1

SM WH→Wbb
SM WH→WWW–SM ZH→Zbb

10

September 29, 2005

-2

110

120

130

140

150

160

170

180mH (GeV)
Figure 2: Summary of the SM higgs search results at the Tevatron. All experimental results
represent the 95% C.L. upper limits.
D∅ with higher precision to be 172.7 ± 2.9 GeV/c2 [4]. Using this result and the current world
average of MW (= 80.410 ± 0.032 GeV/c2 ), the 95% C.L. upper limit on the SM higgs mass was
set to be 186 GeV/c2 by means of the electroweak global ﬁt [5].

4MSSM higgs boson searches

As mentioned in the introduction, the simplest and the most natural extension of the SM is theMSSM. Two higgs ﬁeld doublets which appears in the MSSM framework provide ﬁve physicalhiggs bosons, namely three neutral higgs h0 /H 0 /A0 (= φ) and two charged higgs H ± .
We begin with showing the result on the neutral higgs search through gg/bb̄ → φ → τ τ .
The well-developed algorithm of τ identiﬁcation enabled a study of this channel. A pair-createdτ + τ − was identiﬁed with a signature having one lepton resulting from leptonic decay of τand one isolated narrow jet resulting from hadronic decay of the other τ . We require thelepton ET or pT to be greater than 10 GeV, and the isolated narrow jet to include odd numbertracks, which indicate for them to be originated from one charged track. This τ identiﬁcation
scheme was validated using Z → τ τ events, and we obtained a quite good agreement between
the Monte Carlo simulation and data. In consequence of no signiﬁcant excess in the visible
mass distribution, the upper limits on the production cross section were calculated. From aphenomenological point of view, the MSSM framework can be expressed by a mass of A0 (MA )and a ratio of vacuum expectation value of two higgs doublets (β). The regions excluded by thisanalysis on the MA and tan β plane are shown in Figure 3 for two benchmark cases. Please see[6] for more details on this analysis.
Next we describe the charged higgs search. When the charged higgs mass (MH ± ) is smaller
than the sum of the top quark mass (Mt ) and the bottom quark mass (Mb ), a top quark can
decay to a charged higgs through t → H ± b followed by H ± → τ ν, cs, W bb̄. Existence of these
additional decay channels of a top quark would provide anomaly in the top cross section andthe ﬁnal state components. However, no anomaly was found with the data of 193 pb−1 . This
result was interpreted as constraints of MH ± and tan β. Figure 4 shows excluded regions for thetwo benchmark scenarios of the MSSM. More details can be found in [7].

µ = +200 GeV, M2 = 200 GeV, mg˜ = 0.8 MSUSY–
MSUSY = 1 TeV, Xt = √6 MSUSY (mhmax); MSUSY = 2 TeV, Xt = 0 (no-mixing)µ = -200 GeV, M2 = 200 GeV, mg˜ = 0.8 MSUSY–
MSUSY = 1 TeV,
Xt = √6 MSUSY (mhmax),
Xt = 0 (no-mixing)

100

100

µ<0CDFno mixingmmax
h80

80µ>0mmaxh

CDFmmaxh

CDF Run II 310 pb-1
MSSM Higgs→ττ Search
Preliminary

no mixing

6040tan βtan βDØ
CDF Run II 310 pb-1
MSSM Higgs→ττ Search
Preliminarymmax
h60
402020LEP 2080100120no mixing

no mixing

140160

180

LEP 2

080

200

100

120

140160180200mA (GeV/c2)

mA (GeV/c2)

Figure 3: The excluded regions on the MA - tan β plane for two MSSM benchmark cases.

160160t → H± b search
CDF Run II-1
Excluded 95 %CLmt= 175 GeV/c2∫ L dt = 193 pb

SM Expected

LEP Excluded

80±601010060Assuming H → τν or H → c s only

-112tan( β)102102120100

MSUSY=1000 GeV/c , µ =-200 GeV/c , A t=A b= µ /tan( β), A τ=500 GeV/c
M1=0.498*M 2 , M 2 =200 GeV/c2 , M 3=800 GeV/c2 , M Q=M U=M D=M E =M L=M SUSYCDF Run II Excluded
LEP Excluded80

12010080

LEP (ALEPH, DELPHI,
L3 and OPAL)±±60
102140SM ± 1 σ Expected

212080LEP (ALEPH, DELPHI,
L3 and OPAL)±140
Theoreticallyinaccessible

100

TheoreticallyinaccessibleTheoreticallyinaccessible

120

CDF Run II Excluded

mH+ (GeV/c )140

SM ± 1 σ Expected

2mH+ (GeV/c )

140

160

SM Expected
Theoreticallyinaccessible

160

t → H± b search
CDF Run II-1Excluded 95 %CL
mt= 175 GeV/c2∫ L dt = 193 pb

60

Assuming H → τν or H → c s only-11tan( β)10102MSUSY=1000 GeV/c , µ =-200 GeV/c , A t=A b= 6MSUSY+ µ /tan( β), A τ=500 GeV/c 2
M1=0.498*M 2 , M 2 =200 GeV/c 2 , M 3=800 GeV/c2 , M Q=M U=M D=M E =M L=M SUSY
22±Figure 4: The excluded regions on the MH- tan β plane for two MSSM benchmark cases.

5Future Prospects

The most important key for success of collider experiments is luminosity. As mentioned previously, we are receiving the typical luminosity of 1.5 ∼ 1.7 × 1032 cm−2 s−1 as a peak value and15 ∼ 20 pb−1 as weekly integration. In order to make a realistic estimation of the luminosity
accumulation, a practical strategy for luminosity upgrade was made by the Fermilab accelerator
division in FY04, and they set a “base plan” as a simple extrapolation as of the end of FY04
and a “design plan” as a case all technical challenges are succeeded. The luminosity estimation
based on the above study is 4.1 fb−1 at the end of FY09 for the base plan and 8.2 fb−1 for thedesign plan. It should be noted that we are on the track of the design plan as of January 2006.
On the other hand, we have sensitivities of 95% C.L. exclusion with 2 fb−1 and 3σ evidencewith 4 fb−1 for the SM higgs boson of Mh = 120 GeV/c2 as a result of the precise Monte Carlosimulation. With respect to the MSSM higgs bosons, although it is not easy to tell sensitivities
in detail due to many SUSY parameters, we can say that large region of tan β > 30 on the MA- tan β plane could be explored with 4 ∼ 8 fb−1 .

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