%============================================================================== 
% % mpi08 Worshop proceedings: Individual contribution % 
%============================================================================== 
\documentclass[10pt,here]{mpi08} \usepackage{graphicx} 
\usepackage{cite,./mcite} \def\bibtex{{\sc BiBTeX}} 
\setcounter{footnote}{1} 
%============================================================================== 
% Here comes the document. 
%============================================================================== 
\begin{document} \title{Preparation for forward jet measurements in Atlas} 
\author{Mario Campanelli$^1$\protect\footnote{On behalf of the Atlas collaboration}}
\institute{$^1$University College London}
\maketitle
\begin{abstract}
The Atlas collaboration is defining the strategies for forward physics analyses
with the first data. Most of the cross section at the LHC will involve 
production of particles in the forward direction, and the large rapidity 
coverage of Atlas allows the study of several interesting QCD channels,
both in the framework of diffraction and for studies of underlying event and
QCD evolution.
\end{abstract}

\section{Introduction}
\subsection{Forward physics at the LHC}
The first LHC data will mainly be used for commissioning and calibration, but 
even with small luminosity a large number of events with forward jets will 
be recorded. The LHC detectors aim at covering values of rapidity up to 5, 
much larger than CDF and D0, allow to say something new about forward physics.
Still, most of the particles are produced in the rapidity regions above 5,
so far uninstrumented. A vast program \cite{fp420} is however under way to
extend the coverage of both ATLAS and CMS detectors to rapidities of 10 or
more, using the LHC dipoles as giant spectrometers to measure protons that
remain intact after a diffractive interaction.

\subsection{Forward jet production}
Most of the LHC interactions will involve forward jets final states.
In most of QCD events, jets are produced by fragmentation of coloured quarks
and gluons, and also coloured objects are produced between the jets. So,
in events with forward-backward jets, quite a strong hadronic activity is
present in the forward region.\par
In some cases, final-state jets are produced through the exchange of colourless
particles, like vector bosons, or gluons combining to form a colour-singlet
state (often referred as a pomeron, or odderon depending on its parity
quantum numbers). Exchange of colourless objects has a much smaller cross
section than the exchange of coloured ones, but their characteristic signature
is the presence of a rapidity gap, i.e. a zone of the detector with very little
or absent hadronic activity.
Not all events produced by the exchange of colour singlets will have a rapidity
gap: initial and final state radiation will destroy the gap in the majority of
the cases, and in the literature we usually define the gap survival fraction
as the probability that a colour-singlet event will have a real rapidity
gap. The interesting point is that this fraction is independent of the gap
size, while for events with exchange of coloured objects, the presence of
rapidity gaps is suppressed exponentially as a function of the gap size.
Looking for large rapidity intervals between jets increases the likelihood of
finding events with large gaps, hence the interest in looking for events
with very forward and very backward jets.
\subsection{QCD evolution}
In most of the QCD calculations, the evolution from the hard scattering,
usually calculated using a matrix element, and the soft scale, is done using
the DGLAP \cite{dglap} equation, where gluon splittings are ordered in $k_{T}$ 
and $x$, and sums on $ln(Q^2)$. The BFKL equation \cite{bfkl} performs ordering
in $x$ (and random walk in $k_{T}$) and resummation in $ln 1/x$, therefore it is
more suitable to describe low-x processes like forward-backward jets.
\begin{figure}
\begin{center}
\includegraphics[width=7cm]{gluonladder.eps}
\end{center}
\caption{A Feynman diagram showing a gluon ladder }\label{fig:ladder}
\end{figure}

The resulting description is often depicted as a gluon ladder connecting
quarks from the initial proton (see figure \ref{fig:ladder}. 
When no gluon lines are emitted from the ladder, the gluon ladder behaves as
a colour singlet, and these events will have a rapidity gap in the final
state, i.e. a region of the detector with very little or absent hadronic 
activity.

\section{Previous measurements on hard colour singlet}
Events with two jets separated by rapidity gaps have already been measured at
the Tevatron and at HERA, where events with pure colour singlet exchange
(without initial- or final-state radiation) were measured to be about 1\% of the
total hadronic interactions. In particular a paper from D0 \cite{d0paper}
studied the evolution of the fraction of events with a rapidity gap as a
function of the $\Delta \eta$ between the two jets, up to a rapidity interval
of 6, getting higher results to what expected from Herwig, that also 
incorporates the BFKL approach. It was suggested \cite{cox} that having a 
fixed value of $\alpha_{S}$ (as opposed to a running one) at the vertex between
the pomeron and the quark does a better job in fitting the data, but more data
are needed to solve this issue.

\section{First predictions for the LHC}
The extrapolation of the Tevatron measurements to the LHC energies is not 
obvious, but most of the present models foresee an increase of the survival 
factor (the probability that a rapidity gap event remains intact also after 
initial- and final-state radiation) at LHC energies. This increase is expected 
to be even larger for large gaps, and cross sections are such that a few 
pb$^{-1}$ of data will be sufficient to have a measurement of the survival
factor at the percent level, at least for values of $|\Delta\eta|<8$.
The analysis of rapidity gap events is not easy from the experimental point
of view. To properly define a rapidity gap one should combine calorimeter 
clusters with Et above a certain threshold into mini-jets using the kt
algorithm. Then the total transverse energy in the gap is summed up, and
clusters coming from obvious pileup events are discarded. The analysis
of these events in ATLAS is still ongoing, so the effect of background and
pileup in ``soiling'' the gap is under study. Potentially, the fact that the
fraction of rapidity gap events on the total of hedronic ones has to be
independent on instantaneous luminosity (therefore on the amount of pileup) 
can be a very powerful tool to determine the efficiency of pileup corrections.
One could in fact plot the fraction of gap events as a function of the 
instantaneous luminosity, expecting this fraction to be decreasing as effect of
pileup. Applying pileup corrections, this slope is expected to reduce, and
the amount of this reduction will provide a measurement of the efficiency of
these corrections.
\section{Beyond gaps, M\"uller-Navelet jets}
The gluon ladder does not only predict an increase of events with large
rapidity gaps. In case the gluon ladder also has additional external gluon 
lines, gluon jets will be emitted in the central part of the detector, between
the two main jets. This emission will result in interesting QCD radiation
patterns, and this additional radiation will spoil the back-to back nature of 
the two leading jets. The de-correlation of the azimuthal angle between the
two leading jets is expected to be one of the first measurements with LHC
data, since it does not require too detailed energy calibration. These
de-correlation effects should be already visible for values of $\Delta\eta$
accessible in the LHC experiments, as discussed in \cite{royon}.\par
So far, BFKL has been approximated in MonteCarlo by a Colour Dipole Model 
(CDM)\cite{cdm}, available since years in ARIADNE \cite{ariadne}, widely used
at HERA.\par
A third approach to QCD evolution, the CCFM equation \cite{ccfm} is based on
kt factorisation, angular ordering (instead of kt as for DGLAP), and is a good
approximation of the DGLAP approach at high-Q$^2$ and of BFKL for low x.
This equation is currently implemented in the CASCADE \cite{cascade} code.
Comparison of CDM and CCFM approaches to HERA data did not give conclusive
results, that could on the other hand be obtained from a few days of LHC
running. For instance, the cross section for dijet events separated by $\Delta
\eta$ of at least 2 is of the order of the $\mu$barn. A recent advance has been
the availability of a MonteCarlo code implementing the BFKL formalism 
\cite{jeppe}, even if a proper comparison with data would require interface
with hadronisation, not yet available. 
\section{More diffractive topologies}
So far we have considered events with forward-backward jets, with or without
a rapidity gap in the middle. There are however many more diffractive 
topologies presently under study for the first period of data-taking in ATLAS.
The most studied are single diffraction, where one proton remains intact (and
undetected), and a rapidity gap is present on the same side of the detector.
Another interesting topology is the Central Exclusive Production (CEP), where
the exchange of two colour singlets lead to a final state where both protons
stay intact, and two rapidity
gaps are present, in the forward and backward region of the detector. The
central activity is present in the form of dijets or exclusive final states.
All energy lost by the protons goes in the mass of the central system, and
a precision measurement of their momentum would allow high precision in the
determination of the mass of the central system. A detailed discussion of
the detector upgrades ATLAS (and CMS) are planning to install for the 
determination of the proton momentum loss will be discussed in the
next session.\par
Lacking, at least for the first phase, a dedicated proton tagger, the main
problem to observe CEP with the first LHC data is a valid trigger strategy.
The observable system is quite soft, and the production of jets, dominated
by QCD, will be heavily prescaled at trigger level. Requiring the presence
of rapidity gaps at L1 trigger level is possible in ATLAS using a detector
designed to trigger on minimum-bias events at low luminosity, the Minimum-
Bias Trigger Scintillators (MBTS). They are a set of 32 scintillators,
arranged in two wheels, each covering the rapidity region between 2 and 4.
The aim of this detector is to provide a fast and simple trigger for minimum 
bias events, and due to radiation damage it will have to be removed after
a few years of data taking. In this case, since we are looking at rapidity 
gaps, the MBTS are used as a veto, to select events where no particles are
present in a given rapidity region. It was shown that a veto on both sides
of the MBTS can reduce the QCD rate by a factor 10000, while keeping the
efficiency to CEP of around 65\%. In realistic data-taking conditions, the
MBTS rate is expected to be higher, due to the more radioactive environment,
so realistically both rejection factor and efficiency are expected to be
smaller than these simulated figures.\par
The distribution of the energy lost by the incoming protons (therefore, the
mass of the central system) is on average much smaller than $10^{-2}$ for
diffractive events, while typical values for non-diffractive interactions are
in the 0.1-0.5 range. If no dedicated proton detector is present, we can
estimate the resolution on this variable of the order of 10\%, only using the
information from the central calorimeters. Such a resolution is inadequate
to distinguish a narrow resonance from a much larger background (as it would
be the case for a diffractively-produced Higgs boson), and due to the steeply
falling behaviour of this distribution, also leads to a shift in the measured
mean value. In order to make a precise measurement of CEP processes, it is
necessary to equip the LHC detectors of high-precision proton taggers, like
those proposed to both ATLAS and CMS by the FP420 collaboration\cite{fp420}.
\begin{figure}[H]
\begin{minipage}[h]{0.47\textwidth}
\begin{center}
\includegraphics[width=6cm]{MassTest420_reduced_bb_gg_olap_10ps_mixedlumi_7.5_10.eps}
\end{center}
\end{minipage}
\begin{minipage}{0.47\textwidth}
\begin{center}
\includegraphics[width=6cm]{MassTest420_reduced_bb_gg_olap_5ps_mixedlumi_7.5_10.eps}
\end{center}
\end{minipage}
\caption{The reconstructed mass of the SM Higgs boson A for a time resolution of 10 ps (left) and 5 ps (right)}\label{fig:higgsm}
\end{figure}
\par
\begin{figure}[H]
\begin{center}
\includegraphics[width=9cm]{planarvs3dsili.eps}
\end{center}
\caption{A comparison between 3D silicon (left) and planar geometry (right)}\label{fig:3dsil}
\end{figure}
\par
\section{Forward detectors at the LHC}
Both LHC general-purpose detectors will be equipped by detectors in the
forward region, extending far beyond the coverage of the calorimeters of
about 5. In Atlas, the luminosity monitor Lucid, based on detection of
Cerenkov light, will cover (even if with limited azimuthal coverage for the
first period) a rapidity region down to 6.2, while a zero-degree calorimeter,
located at about 150 meters from the interaction point, will measure neutral
particles emitted almost parallel to the beam direction. None of these
detectors will be however incapable of tagging or measuring the momentum of
protons scattered off diffractive events. Since measuring them is quite 
important, and can be done in an elegant way using the LHC optics as a giant
spectrometer, a group of physicists, most of whom from the fp420 collaboration
\cite{fp420}, is proposing to install two detectors at 220 and 420 meters
from the Atlas interaction point. The goal is to measure with high precision 
the position of the protons diffracted from the beam (and from that their
momentum, using the LHC dipoles as a giant spectrometer), as well as their
time of flight, in order to distinguish particles coming from different
vertexes in a high-pileup situation.\par
The stringent radiation hardness and speed requirements of the position
detectors required the development of a new technology. 3D silicon detectors
(see figure \ref{fig:3dsil}), the result of a long R\&D work, have several
advantages with respect to the planar geometry: they work with a smaller
depletion voltage, are more radiation hard and are faster since the drift
is shorter. They can operate at few mm from the beam line, in both the 220
and 420 meter location. The requirements on the timing detectors are also 
very stringent. The problem comes from the fact that at high-pileup conditions
a Central Exclusive Production event can be perfectly faked by the overlap
of a soft-QCD production event plus two single-diffractive interactions.
The only way to separate them is due to the fact that these overlapping
events come from different vertexes, so if the vertex position can be
determined with a resolution of 2-3 mm, a sufficient background rejection
can be obtained. While such a resolution is easy to reach using tracks for
the central system, the only way to have good vertex resolution for the forward
protons is to have a very precise (10 ps resolution) time of flight detector.
So far, two technologies have been proposed, a gas tube with a mirror at the
end to detect Cerenkov light, and an array of quartz detectors, that also can
focus Cerenkov light into a multi-channel plate photomultiplier.
So far, test-beam results indicate that a resolution of 10-20 ps can be 
obtained by the gas approach, while 20-30 ps can be reached by the gas 
detector, that on the other hand has a higher light yield and can be 
spatially segmented. R\&D for timing detectors is still going on, and 
maybe a combination of the two technology can offer the advantages of both.
To see how timing resolution can be important for the whole project, figures
\ref{fig:higgsm} show the expected peak of a possible MSSM Higgs boson A
($m_A$ = 120 GeV, tan$\beta$=40, $\sigma (h\to bb)$ = 17.9 fb) with time
resolutions of 10 and 5 ps.


\subsection{Conclusions}
Diffractive and forward physics, due to their large cross-section and need 
for a low-pileup
environment, will play a large role in the LHC startup. The main research 
topics will be:\begin{itemize}
\item the study of forward jets, both with and without rapidity gaps. The
first analysis will measure the soft survival factor, and help understanding
forward jets and rapidity gaps, while the second will discriminate between
different QCD evolution schemes. These studies will require a few tens of
pb$^{-1}$ of data
\item single diffraction, with one undetected proton and a matching rapidity
gap, will provide complementary measurements on the interface between the
jets and the gap. Its study will require a few hundreds of pb$^{-1}$.
\item Central exclusive production, with two rapidity gaps and a soft central
system, will also help understanding diffractive PDF's, Sudakov suppression
factors, and discriminate among theoretical models. A few hundreds of pb$^{-1}$
are needed for a complete study of these events
\end{itemize}
For the future, ATLAS is planning to install a four-station proton tagger
station to measure the momentum loss of the forward protons, therefore the
mass of the central system, and the accurate time of flight, to distinguish
genuine diffractive events from pileup background. Installation of these 
detectors, still under approval, is foreseen by 2013-2014.

%------------------------------------------------------------------------------
%       Bibliography
%------------------------------------------------------------------------------
\begin{footnotesize}
\bibliographystyle{mpi08} 
{\raggedright
\bibliography{mpi08}
}
\end{footnotesize}
\end{document}
