M. Oreglia
for the LEP Higgs Working Group
fermio.tex
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%  Title :   Searches for Higgs Bosons Decaying into Photons
%  Authors:  Mark Oreglia for the LEP Higgs Working Group
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\newcommand{\Author}    {M. Oreglia, for the LEP Higgs Working Group}
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% these don't need explicit $

\newcommand{\ee}{\mbox{${\mathrm{e}}^+ {\mathrm{e}}^-$}}
\newcommand{\tautau}{\mbox{$\tau^+\tau^-$}}
\newcommand{\mm}{\mbox{$\mu^+\mu^-$}}
\newcommand{\ellell}{\mbox{$\ell^+\ell^-$}}
\newcommand{\eeff}     {\mbox{$\mathrm e^+e^- f \bar{\mathrm f}$}}

\newcommand{\hwa}        {\mbox{$\mathrm{H^\pm}\ra\mathrm{W^{*\pm}A^0}$}}
\newcommand{\qq}         {\mbox{$\mathrm{q}\bar{\mathrm{q}}$}}
\newcommand{\bb}         {\mbox{$\mathrm{b}\bar{\mathrm{b}}$}}
\newcommand{\ff}         {\mbox{$\mathrm{f}\bar{\mathrm{f}}$}}
\newcommand{\lpair}      {\mbox{$\ell^+\ell^-$}}
\newcommand{\nunu}       {\mbox{$\nu\bar{\nu}$}}
\newcommand{\mZ}         {\mbox{$m_{\mathrm{Z}}$}}
\newcommand{\mH}         {\mbox{$m_{\mathrm{H}}$}}
\newcommand{\mh}         {\mbox{$m_{\mathrm{h}}$}}
\newcommand{\mA}         {\mbox{$m_{\mathrm{A}}$}}

\newcommand {\Ho}        {\mbox{$\mathrm{H}^{0}$}}
\newcommand {\Ao}        {\mbox{$\mathrm{A}^{0}$}}
\newcommand {\ho}        {\mbox{$\mathrm{h}^{0}$}}
\newcommand {\Zo}        {\mbox{$\mathrm{Z}^{0}$}}
\newcommand{\Hpm}{\mbox{$\mathrm{H}^{\pm}$}}
\newcommand{\Hp}{\mbox{$\mathrm{H}^+$}}
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\newcommand{\mHpm}{\mbox{$m_{\mathrm{H}^{\pm}}$}}
\newcommand{\nbar}{\mbox{$\overline{\nu}$}}
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\newcommand{\Zs}         {\mbox{${\mathrm{Z}}^{*}$}}
\newcommand{\Zgs}        {\mbox{$\mathrm{(Z/\gamma)}^{*}$}}
\newcommand {\Wpm}       {\mbox{$\mathrm{W}^{\pm}$}}
\newcommand{\MH}{M_{\mathrm H}}
\newcommand{\MZ}{M_{\mathrm Z}}
\newcommand{\qqbar}{{\mathrm q}\bar{\mathrm q}}
\newcommand{\nn}{\nu \bar{\nu}}

\newcommand{\gaga}       {\mbox{$\gamma\gamma$}}
\newcommand{\WW}         {\mbox{$\mathrm{W}^+\mathrm{W}^-$}}


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           {\sim}$}\;}
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\newcommand{\Rbal}      {\mbox{$R_{\mathrm{bal}}$}}
\newcommand{\Ebeam}       {E_{\mathrm{beam}}} 
\newcommand{\Mrec}      {M_{\mrm{recoil}}}
\newcommand{\mgg} {$M_{\gamma \gamma}$}
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\newcommand{\PhysLett}  {Phys.~Lett.}
\newcommand{\PRL}       {Phys.~Rev.\ Lett.}
\newcommand{\PhysRep}   {Phys.~Rep.}
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\newcommand{\NPhys}     {Nucl.~Phys.}
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\newcommand{\ZPhysC}[1]    {Z. Phys. {\bf C#1}}
%\newcommand{\EurPhysC}[1]    {Eur. Phys. Journal {\bf C#1}}
\newcommand{\EurPhysC}[1]    {Eur. Phys. J. {\bf C#1}}
\newcommand{\PhysLettB}[1] {Phys. Lett. {\bf B#1}}
\newcommand{\CPC}[1]      {Comp.\ Phys.\ Comm.\ {\bf #1}}
\def\NIM                {\mbox{Nucl. Instr. Meth.}}
\newcommand{\NIMA}[1]   {\NIM\ {\bf A{#1}}}
\def\etal{\mbox{{\it et al.}}}
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\newcommand{\OPALColl}  {OPAL Collab.}
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\begin{document}
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%=======================================================================
%       Title Page
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\begin{titlepage}
\centerline{\Large EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH}

\begin{flushright}
      LHWG Note 2001-08 \\
      ALEPH 2001-059 CONF 2001-039 \\
      DELPHI 2001-116 CONF 539 \\
      L3 Note 2701 \\
      OPAL Technical Note TN695 \\
      July 2, 2001 \\   
\end{flushright}

\bigskip
\bigskip
\bigskip
\bigskip
\bigskip

\begin{center}{\LARGE\bf Searches for Higgs Bosons Decaying into
Photons: Preliminary Combined Results Using LEP Data Collected at
Energies up to 209~GeV}
\end{center}
\bigskip
\begin{center}{\LARGE The ALEPH, DELPHI, L3 and OPAL Collaborations\\
The LEP Higgs Working Group}
\end{center}

%%\vspace{0.1cm} 
%%\begin{center}
%% {\Large {The Photonic Higgs Group}} \\
%%{\bf Contacts:  
%%Sofia Andringa-Dias (DELPHI),
%%Mark Oreglia (OPAL),
%%Aura Rosca/Chris Tully (L3),
%%Andre Tilquin (ALEPH)
%%}
%%\end{center}

%\begin{center}
% {\bf Comments to: Mark.Oreglia@cern.ch } \\
% {\bf by Friday, 29 June 2001 please.}
%\end{center}
%
%\bigskip
%
%\begin{center}
% {\Huge {\bf Draft Version \Version}}\\
% {\large \today }\\
%\end{center}

\bigskip\bigskip

\bigskip
\begin{center}{\large  Abstract}\end{center}
This note describes preliminary LEP-combined results of searches 
for Higgs bosons decaying into photons. 
The analyses use data collected at 
$\sqrt{s} \approx $88$-$209~GeV.
Using the combined data for \Ho\ra\gaga,
a lower bound of 108.2~GeV is set at the 95\% confidence level 
for the ``benchmark'' fermiophobic Higgs boson 


\bigskip
\bigskip
%\bigskip
%\bigskip
%
\begin{center}
  {\Large
   THE RESULTS QUOTED IN THIS NOTE ARE PRELIMINARY}
\end{center}

\end{titlepage}



%
%  =============== INTRODUCTION =================
%
\newpage
\section{Introduction}

In the Standard Model (SM), the branching fraction
for \ho\ra\gaga\
is too small to permit its observation at LEP energies.
However, in particular formulations of 
2-Higgs Doublet Models (2HDM)
and other models,
the Higgs coupling to fermions can be small
and the Higgs bosons therefore decay preferentially to pairs of bosons.
These are the so-called ``fermiophobic'' Higgs bosons
(see, e.g., references~\cite{Akeroyd,Santos}).

The fermiophobic models are indeed parameter dependent,
but a large class of the models has near-Standard Model production
strength.
Therefore, we establish a ``benchmark'' fermiophobic model
defined to be the Standard Model production rates and channels,
but with the fermionic channels closed.
%
To model Higgs boson production 
the four LEP experiments are using the code 
HZHA~\cite{HZHA3} and/or HDECAY~\cite{HDECAY}. 
The code 
HDECAY gives a slightly lower di-photon branching fraction than HZHA
(the difference in mass limits near 100~GeV is about 0.6~GeV).  
In the interest of conservatism, we use
the HDECAY branching fractions in this LEP-wide combination.
The bosonic branching fractions obtained from HDECAY in the benchmark
model are shown in Figure~\ref{hgg-br}.

To obtain the limits, the LEP Higgs Working Group uses the frequentist confidence levels
generated by the ``ecl'' package~\cite{ecl}; three of the LEP
collaborations use different statistical methods, which have been
shown to agree with the LHWG method to within 0.5~GeV on all mass limits. This
technique includes the estimated systematic errors and the expected signal shape in
setting test-mass dependent limits on Higgs boson decays to photon pairs.

The LEP limits on the fermiophobic Higgs boson mass will be shown to
exceed 100~GeV, where Figure~\ref{hgg-br} indicates preferred decay
into WW.  One LEP experiment~\cite{L3WW} has set limits on the WW
mode, but this channel is not considered in the present combination.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
\section{\boldmath Searches for \ho\Zo\ with \ho\ra\gaga }

The 
ALEPH~\cite{ALEPH}, 
DELPHI~\cite{DELPHI}, 
L3~\cite{L3}, and 
OPAL~\cite{OPAL}
analyses are described in journal
articles or CERN preprints.
All of the LEP experiments search for hadronic, leptonic, and missing
energy (neutrino) decay modes of the associated Z boson in the
production channel \ee \ra \ho\Zo.  
The ALEPH experiment does not
discriminate between Z decay modes; rather, it performs a ``global''
analysis which focusses on identifying the di-photon state.  DELPHI,
L3, and OPAL seek to identify the 
hadronic (\Zo\ra\qq), 
leptonic (\Zo\ra\lpair), 
and 
missing energy (\Zo\ra\nunu) 
classes. 

The LEP data from energies just below the Z resonance to the highest
LEP-2 energy of 209 GeV can be used for this search.  
%ALEPH and OPAL
%have analyses which include all LEP data, however the ALEPH energies
%below 192~GeV are not currently used in the LEP combination.
The energies and Z decay modes used by the 4 LEP experiments are
summarized in Table~\ref{T:hgg1}.


%
% table
%
\begin{table}[htbp]
\begin{center}
{\footnotesize
\begin{tabular}{|l||c|c|c|c||c|}\hline
        & ALEPH & DELPHI & L3  & OPAL & SUM \\ 
\hline\hline

Modes     & global & \qq,\lpair,\nunu\ & \qq,\lpair,\nunu\ & \qq,\lpair,\nunu\ & \\
$\sqrt{s}$ (GeV) & 192-209 & 189-209 & 189-209 & 88-209 & 88-209 \\
Candidates      & 10   & 47   & 64   & 184   & 305 \\
Background      & 10.8 & 42.2 & 69.5 & 185.7 & 308.2\\
Benchmark limit (GeV) & 104.4 & 103.6 & 104.1 & 104.8 & 108.2 \\
Expected limit (GeV)  & 104.6 & 105.1 & 104.9 & 105.2 & 109.0 \\
\hline
\end{tabular}
}
\end{center}
  \caption[Datasets and Limits]
  {
    Parameters of the photonically-decaying Higgs searches.
95\% confidence level lower limits on the mass of a benchmark
  fermiophobic Higgs boson are shown.
}
  \label{T:hgg1}
\end{table}


The candidates passing the selection cuts for all three decay topologies
are shown in Table~\ref{T:hgg1};
their rate is consistent with the background calculated from SM
physics generators. 
The selected events are used to set an upper limit
on the di-photon branching ratio of a Higgs particle having SM
production rate.  
Figure~\ref{hgg-mgg} showns the di-photon mass for all candidate
events. The distribution appears to be well modelled by the SM background processes.

Figures~\ref{hgg-adlo} and~\ref{hgg-s95} show the 95\% CL upper limit for the
di-photon branching ratio obtained by combining the candidate events,
where the SM $\ho \mrm Z^{0(*)}$ production cross-section
is assumed at each centre-of-mass energy.
Also shown in the Figures (and in Figure~\ref{hgg-br}) is the $\ho \ra \gaga$ branching
ratio in the Standard Model computed using HDECAY~\cite{HDECAY} 
with the fermionic couplings switched off.
%
Note that the limits only include the ALEPH data for $\sqrts \ge$ 192~GeV, 
and the L3 inputs to the LHWG only cover the mass range 50-120~GeV.
%
A 95\% CL lower mass limit 
for such benchmark fermiophobic Higgs bosons 
is set at 108.2~GeV, where the predicted branching ratio 
crosses the upper-limit curve.  The median limit one would expect to obtain in
an ensemble of experiments in the absence of a signal is 109.0~GeV;
if HZHA is used instead of HDECAY to calculate the Higgs boson
branching fractions, the lower mass limits are raised by 0.6~GeV.


%\newpage
%%%%%%%%%%%% REFERENCES %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%=======================================================================
%     References
%=======================================================================
\begin{thebibliography}{99}
%

\bibitem{Akeroyd} 
      A. G. Akeroyd, \PhysLettB{368} (1996) 89.

\bibitem{Santos}
      L.~Brucher, R.~Santos, Eur. Phys. J. {\bf C12} (2000) 87.

\bibitem{HZHA3} HZHA generator: P.~Janot, in {\em Physics at LEP2,}
               edited by G.~Altarelli, T.~Sj\"{o}strand and
               F.~Zwirner, CERN 96-01 Vol.~2  p.309. \\
For HZHA3 and HZHA2, see {\em http://alephwww.cern.ch/$\sim$janot/Generators.html}.


\bibitem{HDECAY}
      A. Djouadi, J. Kalinowski and M. Spira, \CPC{108} (1998) 56.     

\bibitem{ecl}
      T. Junk, \NIM{A434} (1999) 435.

\bibitem{L3WW}
      L3 collaboration, L3 Notes 2649 (5 March 2001) and 2685 (27 June
2001).

\bibitem{ALEPH}
      ALEPH collaboration,
      \PhysLettB{487} (2000) 241.

\bibitem{DELPHI}
      DELPHI collaboration,
      \PhysLettB{507} (2001) 89;
      updated:  DELPHI 2001-073 CONF 501 (June 2001).

\bibitem{L3}
      L3 collaboration,
      \PhysLettB{489} (2000) 115;
      L3 Note 2652 (8 March 2001).

\bibitem{OPAL}
      OPAL collaboration,
      \PhysLettB{464} (1999) 311;
      OPAL Physics Note PN 468 (2 November 2000).


\end{thebibliography}




%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% figures 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
\clearpage
\newpage
    \begin{figure}[!htb]
        \vspace{0.8cm}
        \begin{center}
           \resizebox{\linewidth}{!}{\includegraphics{hgg_fig1.eps} }
        \caption[hgg-br]{    
             Branching fraction of benchmark fermiophobic Higgs boson
        into boson pairs.
        \label{hgg-br} }
        \end{center}
    \end{figure}

%
\clearpage
\newpage
    \begin{figure}[!htb]
        \vspace{0.8cm}
        \begin{center}
           \resizebox{\linewidth}{!}{\includegraphics{hgg_fig2.eps} }
        \caption[hgg-mgg]{    
             Distribution of di-photon masses for the LEP experiments
        combined.  The expected background from all SM sources is indicated
        by the lightly shaded histogram.  
        The signal expected for a
        benchmark fermiophobic Higgs boson of mass 100 GeV
        is shown by the dark shaded histogram.
        \label{hgg-mgg} }
        \end{center}
    \end{figure}

%
\clearpage
\newpage
    \begin{figure}[!htb]
        \vspace{0.8cm}
        \begin{center}
           \resizebox{\linewidth}{!}{\includegraphics{hgg_fig3.eps} }
        \caption[hgg-adlo]{    
             Individual experimental limits on fermiophobic Higgs
        bosons for the photonic decay mode. The 95\% confidence level upper limit on the di-photon
        branching fraction is shown as a function of Higgs mass;
        the dark shaded regions are excluded. 
        Also
        shown (dotted line) is the branching fraction obtained for the
        benchmark fermiophobic model.

        \label{hgg-adlo} }
        \end{center}
    \end{figure}


%
\clearpage
\newpage
    \begin{figure}[!htb]
        \vspace{0.8cm}
        \begin{center}
           \resizebox{\linewidth}{!}{\includegraphics{hgg_fig4.eps} }
        \caption[hgg-s95]{    
             Combined LEP experimental limits on Higgs
        bosons (of Standard Model production cross section)
        decaying into di-photons. 
        The 95\% confidence level upper limit on the di-photon
        branching fraction is shown as a function of Higgs mass. Also
        shown (dotted line) is the branching fraction obtained for the
        benchmark fermiophobic model. The median expected limits and
        the $\pm\sigma$ confidence level region are denoted by the dashed curves.
        \label{hgg-s95} }
        \end{center}
    \end{figure}



\end{document}

hep-ex0107035.extracted_bib
\begin{thebibliography}{99}
%

\bibitem{Akeroyd} 
      A. G. Akeroyd, \PhysLettB{368} (1996) 89.

\bibitem{Santos}
      L.~Brucher, R.~Santos, Eur. Phys. J. {\bf C12} (2000) 87.

\bibitem{HZHA3} HZHA generator: P.~Janot, in {\em Physics at LEP2,}
               edited by G.~Altarelli, T.~Sj\"{o}strand and
               F.~Zwirner, CERN 96-01 Vol.~2  p.309. \\
For HZHA3 and HZHA2, see {\em http://alephwww.cern.ch/$\sim$janot/Generators.html}.


\bibitem{HDECAY}
      A. Djouadi, J. Kalinowski and M. Spira, \CPC{108} (1998) 56.     

\bibitem{ecl}
      T. Junk, \NIM{A434} (1999) 435.

\bibitem{L3WW}
      L3 collaboration, L3 Notes 2649 (5 March 2001) and 2685 (27 June
2001).

\bibitem{ALEPH}
      ALEPH collaboration,
      \PhysLettB{487} (2000) 241.

\bibitem{DELPHI}
      DELPHI collaboration,
      \PhysLettB{507} (2001) 89;
      updated:  DELPHI 2001-073 CONF 501 (June 2001).

\bibitem{L3}
      L3 collaboration,
      \PhysLettB{489} (2000) 115;
      L3 Note 2652 (8 March 2001).

\bibitem{OPAL}
      OPAL collaboration,
      \PhysLettB{464} (1999) 311;
      OPAL Physics Note PN 468 (2 November 2000).


\end{thebibliography}