%================================================================ % LaTeX file with preferred layout for the contributed papers to % the ICHEP Conference 98 in Vancouver % process with: latex hep98.tex % dvips -D600 hep98 %================================================================ \documentclass[12pt]{article} \usepackage{epsfig} \usepackage{amsmath} \usepackage{hhline} \usepackage{amssymb} \usepackage{times} \renewcommand{\topfraction}{1.0} \renewcommand{\bottomfraction}{1.0} \renewcommand{\textfraction}{0.0} \newlength{\dinwidth} \newlength{\dinmargin} \setlength{\dinwidth}{21.0cm} \textheight23.5cm \textwidth16.0cm \setlength{\dinmargin}{\dinwidth} \setlength{\unitlength}{1mm} \addtolength{\dinmargin}{-\textwidth} \setlength{\dinmargin}{0.5\dinmargin} \oddsidemargin -1.0in \addtolength{\oddsidemargin}{\dinmargin} \setlength{\evensidemargin}{\oddsidemargin} \setlength{\marginparwidth}{0.9\dinmargin} \marginparsep 8pt \marginparpush 5pt \topmargin -42pt \headheight 12pt \headsep 30pt \footskip 24pt \parskip 3mm plus 2mm minus 2mm %===============================title page============================= % Some useful tex commands % \newcommand{\GeV}{\rm GeV} \newcommand{\TeV}{\rm TeV} \newcommand{\pb}{\rm pb} \newcommand{\cm}{\rm cm} \newcommand{\hdick}{\noalign{\hrule height1.4pt}} \newcommand{\ra}{\rightarrow} \newcommand{\ccb}{c\bar{c}} \newcommand{\bbb}{b\bar{b}} \newcommand{\pbp}{\bar{p}p} \newcommand{\epem}{e^+e^-} \begin{document} \pagestyle{empty} \begin{titlepage} \noindent \begin{center} \begin{small} \begin{tabular}{llrr} Submitted to & & & \epsfig{file=/h1/www/images/H1logo_bw_small.epsi ,width=2.cm} \\[.2em] \hline \multicolumn{4}{l}{{\bf 31st International Conference on High Energy Physics, ICHEP02}, July~24,~2002,~Amsterdam} \\ & Abstract: & {\bf 1013} &\\ & Parallel Session & {\bf 5} &\\ \hline & \multicolumn{3}{r}{\footnotesize {\it www-h1.desy.de/h1/www/publications/conf/conf\_list.html}} \\[.2em] \end{tabular} \end{small} \end{center} \vspace*{2cm} \begin{center} \Large {\bf Beauty Production in Deep Inelastic {\boldmath ep} Scattering} \vspace*{1cm} {\Large H1 Collaboration} \end{center} \begin{abstract} \noindent We report the first observation of beauty production in deep inelastic $ep$ scattering (DIS). Hadrons with $b$~fla\-vour are observed with the H1 detector at HERA through their semi-leptonic decay. The cross section is extracted using the distributions of the impact parameter and of the transverse momentum of muons relative to jets in the kinematic range $22$~GeV. The result, $\sigma = [39\,\pm\,8\,(stat.)\,\pm 10\,(syst.)\,]\,{\rm pb}$ is found to be significantly % next-to-leading order above NLO QCD predictions. %We present the first measurement of beauty production in DIS. %The analysis is performed with a sample of DIS events with 2 jets %($E_T>5\,$ GeV) accompanied by a muon which is identified in %the instrumented iron and well measured in the central silicon vertex %detector. The contribution of $b$-events to the sample is determined with %a likelihood fit to the two-dimensional distribution %in the impact parameter and the muon transverse momentum relative to the %jet axis, as applied previously in the photoproduction regime. %The measured cross section is found to be %significantly larger than predicted by LO and NLO QCD calculations. \end{abstract} \end{titlepage} \pagestyle{plain} Beauty production has not yet been observed in deep inelastic scattering (DIS). The process is considered to be reliably calculable in perturbative QCD since the $b$ quark mass sets a hard scale. Particular interest in confronting the theory with experiment in a yet unexplored production environment stems from the fact that $b$ cross sections are found in excess of QCD expectations in $\bar{p}p$~\cite{hadronb}, $\gamma p$~\cite{h1openb,blto,zeusopenb} and, recently, $\gamma\gamma$~\cite{ggb} interactions, where the calculations follow the same principles. Moreover, since charm accounts for about a quarter of the inclusive DIS rate in the HERA domain~\cite{f2ch1zeus}, the proton structure analysis of inclusive DIS data~\cite{f2h1zeus} relies heavily on the understanding of heavy quark production, for which $b$ production constitutes a new testing ground. In electron proton collisions, $b$ quarks are predominantly produced, in QCD, via the interaction of a photon coupling to the incoming electron with a gluon in the proton by forming a quark-anti-quark pair. The case of small photon virtuality, $Q^2<1$~GeV$^2$ corresponds to photoproduction ($\gamma p$). The DIS case of larger $Q^2$ is complementary, because resolved contributions involving the partonic structure of the photon are expected to be suppressed~\cite{grs}. Thus theoretical uncertainties due to imperfect knowledge of hadronic structures are smaller in DIS than in the other reactions. The result presented here is an extension of our measurement of the beauty production cross section~\cite{blto}, from the photoproduction regime to DIS. The analysis starts with a sample of DIS events collected in 1997. The same reconstruction and selection procedures as in~\cite{blto} are used to obtain a dijet event sample with a muon identified in the instrumented iron and well measured in the central silicon tracker (CST)~\cite{cst}. Here the electron is scattered into the main detector. Its signature is used in the trigger which requires electromagnetic energy deposition signals from the backward calorimeter SpaCal in coincidence with tracking information from proportional and drift chambers. The integrated luminosity corresponds to ${\cal L}=10.5$~pb$^{-1}$. The selection of DIS events follows the methods described in~\cite{gluon}. The scattered electron candidate reconstructed in the SpaCal must be detected with a polar angle $\theta_{e'}<177^o$ relative to the incoming proton momentum vector and is required to have an energy $E_{e'} >$ 9 GeV. The DIS kinematic variables $Q^2$ and $y$ are reconstructed from the scattered electron. The cross section is determined for the visible kinematic range $22$ GeV and $35^\circ<\theta(\mu)<130^\circ$. $p_T(\mu)$ and $\theta(\mu)$ denote the transverse momentum and the polar angle of the muon with respect to the beam axis. 168 events are selected which contain $N_{\mu} = 171$ muon candidates. The composition of the sample, in terms of muons from $\bbb$ events, $\ccb$ events, or background from misidentified hadrons (``fake muons'') is analyzed using AROMA Monte Carlo~\cite{aroma} event samples as in~\cite{blto}. The hadron sample used to model the fake muon background is selected using the same requirements as for the signal, except for the muon identification, and using the same trigger for DIS events. %This is in contrast to the photoproduction case, %where the trigger for the signal is based on the muon signature %in the instrumented iron and cannot be used for this purpose. In the photoproduction case, it has been shown that the impact parameter $\delta$ and the transverse momentum $p_T^{rel}$ relative to the jet axis provide independent and consistent signatures for $b$ production. The analysis of the smaller DIS sample relies on the combination of the two observables, which also provided the most precise photoproduction result. %The likelihood fit of the $\bbb$, $\ccb$ and fake muon %reference spectra to the two-dimensional %distribution in $\delta$ and $p_T^{rel}$ %yields a $\bbb$ fraction of $f_b = (43\pm 8)\,\%$. A likelihood fit of $\bbb$, $\ccb$ and fake muon reference spectra to the two-dimensional distribution in $\delta$ and $p_T^{rel}$ adjusts the relative weights of all three components in the data. It yields a $\bbb$ fraction of $f_b = (43\pm 8)\,\%$. The projections of this distribution are shown in Fig.~\ref{fig:disptdelta} together with the decomposition from the fit. The distributions of both variables are well described. The need for a $\bbb$ component is evident from the lifetime based signature as well as from the $p_T^{rel}$ spectrum. % % \begin{figure}[t] \begin{center} \unitlength5mm \begin{picture}(24,18) \put(-4.,-0.){\epsfig{figure=H1prelim-01-072.fig1a.eps,height=9cm}} \put(12.,-0.){\epsfig{figure=H1prelim-01-072.fig1b.eps,height=9cm}} % \end{picture} \caption{\label{fig:disptdelta} Distributions of the muon impact parameter (left) and transverse momentum relative to the jet axis (right), with decomposition from the likelihood fit.} \end{center} \end{figure} % % % %\begin{figure}[t] %\begin{center} %\unitlength5mm %\begin{picture}(24,20) %\put(0., 0.){\epsfig{figure=dis_ptrel.eps,height=12cm}} %% %\end{picture} %\caption{\label{fig:ptrel} %Impact parameter distribution and decomposition from the likelihood fit.} %\end{center} %\end{figure} % The charm and fake muon fractions are varied independently in the fit, but have large, correlated errors. The fit yields $f_{fake} = (11 \pm 22)\,\%$. Within the large error this is consistent with the expectation using fake probabilities determined in Monte Carlo simulations to reweight the hadron track sample, $f_{fake} = 43\,\%$. Fixing the fake muon fraction to this value in the fit changes the result for $f_b$ by less than half the statistical error. %The same holds true if $p_T^{rel}$ alone is used in this constraint fit. The fits to the one-dimensional distributions were performed as cross checks, as in~\cite{blto} and yield consistent results. %Due to the small statistics, one cannot expect to obtain a reliable %independent measurement using a free fit to the impact parameter $\delta$ %alone, as was possible in photoproduction. The sources of systematic uncertainty are the same as in the photoproduction case, except for a small contribution ascribed to the electron reconstruction. The dominant contribution is due to the Monte Carlo Model used to extrapolate the cross section. The biggest experimental errors, arising from the stability with respect to variations of the impact parameter analysis and the change of the jet energy scale, % and the Monte Carlo Model used to extrapolate the cross section were checked separately for the DIS sample and confirmed. %The cross section in the visible range %is defined as in~\cite{blto,h1openb} as %$\sigma = f_b N_{\mu} / (2\epsilon{\cal L})$, %where $\epsilon$ denotes the efficiency for a $\bbb$ event with a muon %in the visible range to fulfill the selection requirements. The cross section for the production of muons from $b$ decays is defined as in~\cite{h1openb} as \[\sigma_{ep}^{vis} (ep\ra bX\ra\mu X)= f_b N_{\mu} / (2\epsilon{\cal L})\, ,\] where $\epsilon$ denotes the efficiency for a $\bbb$ event with a primary or secondary muon in the visible range to fulfill the selection and trigger requirements. The factor of 2 accounts for the fact that the experiment measures muons from $b$ and $\bar{b}$ decays. We find \[ \sigma_{ep}^{vis} = [\;39\;\pm\;8\;(stat.)\; \pm 10\;(syst.)\;]\;{\rm pb}\ . \] This measurement can be directly compared to a NLO QCD calculation using the standard HVQDIS program~\cite{hvqdis} with a Peterson fragmentation function~\cite{peterson} used to scale the quark momenta in order to obtain $b$ hadron momenta. The distributions were then folded with a lepton spectrum extracted from the aroma Monte Carlo generator. The GRV98 parton densities~\cite{grv98} are used, the $b$ quark mass was set to 4.75 GeV, and the renormalization and factorization scales to $\mu = \sqrt{Q^2+4m_b^2}$. The Peterson fragmentation parameter to $\epsilon_b = 0.0033$ as obtained from a fit of a fixed order QCD calculation~\cite{olearimerged} in the same (massive) scheme to the $b$ production spectrum~\cite{alephb} measured in $\epem$ annihilation. The QCD result is $(11\pm 2)$ pb, where the error has been estimated by varying the scale $\mu$ by factors of two, $\epsilon_b$ between 0.0016 and 0.0069, and $m_b$ between 4.5 and 5 GeV. The $b$ mass has the strongest influence ($\pm 11\,\%$). The AROMA prediction is 9~pb. A LO calculation with the CASCADE program~\cite{cascade} based on the CCFM evolution equation yields 15~pb. % % --- HERA b % \begin{figure}[tbp] \begin{center} \unitlength5mm \begin{picture}(24,19) \put(1.,-0.3){\epsfig{figure=H1prelim-01-072.fig2.eps,height=10cm}} % %\put(6.,2.){$\underbrace{\;\;\;\;\;\;\;\;\;\;\;\;}$} %\put(4.7,0.5){\large\sf Q$^2<1\,$GeV$^2$} \end{picture} \caption{\label{fig:herab} Ratio of measured $b$ production cross sections at HERA over theoretical expectation, as a function of $Q^2$. The inner (outer) error bars represent the statistical (total experimental) error, the shaded band the theoretical uncertainty.} \end{center} \end{figure} % We summarize the HERA results as a function of $Q^2$ in Fig.~\ref{fig:herab}. Displayed is the ratio of measured visible cross sections over theoretical expectations, which for the photoproduction case have been calculated~\cite{h1openb,zeusopenb} using the FMNR program~\cite{frixi}. %\footnote{The result of the NLO QCD calculation for the visible %photoproduction cross section given in our first publication~\cite{h1openb} %could not be reproduced. %The measured value of %$\sigma_{ep}^{vis}= %[176\;\pm\;16\;^{+27}_{-17}\; ] \;{\rm pb}$ %must be compared with the NLO QCD expectation of $(48\pm 5)$ pb %obtained using the FMNR program~\cite{frixi} with a %Peterson fragmentation function. %The extrapolated total $ep$ and $\gamma p$ cross sections in~\cite{h1openb} %scale accordingly.} The ratio is consistent with being independent of $Q^2$; the discrepancy between data and theory is thus not a feature of the photoproduction regime alone. \newpage In conclusion, the $b$ cross section in DIS is measured for the first time and is found to be above QCD expectations. 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