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Proofread my QCHS7 proceedings paper – full-size draft October 31, 2006

Posted by dorigo in language, physics, science.

Ok, I just got a postscript version of my draft done… It still misses a couple of figures and I think it has still many errors buried within the text. Plus, I need to check several numbers I inserted by heart in the text so far… Any hardware expert who can check the resolutions I quoted ?

Anyway, I will appreciate any further comments you may have. I have put a copy of the paper in http://www.pd.infn.it/~dorigo/dorigo_qchs.ps

However, for your convenience I paste here the text of the two last sections as well -the ones following the piece I already posted yesterday. If I feel you have contributed to the clarity of the text I will be glad to add your name in the Acknowledgements section…


Searches for the Standard Model Higgs boson

The search for the Higgs boson at the Tevatron is carried out by looking for its two main
decay signatures, depending on the particle mass:
if $M_H<135$ GeV the dominant decay is $H \to b\bar{b}$, while at higher masses the
$H \to WW$ decay provides the most promising signature.
Both CDF and D\O\ search their datasets for $WH$ and $ZH$ associated
production with a low mass higgs boson decaying to a pair of $b$-quark jets,
while the vector boson is tagged by the reconstruction of two charged leptons
(for $Z \to ee$ or $\to \mu \mu$), a lepton and missing transverse energy (to select
$W \to e \nu$ or $\to \mu \nu$), or missing transverse energy alone (for $Z\to \nu \nu$
decays). The two critical parameters affecting signal significance are
the efficiency of $b$-quark jet identification -performed through the reconstruction
of a secondary vertex in the jet- and the resolution of a reconstructed dijet mass peak.
The dijet mass distribution is studied in search for an excess over backgrounds, which
are mainly due to real vector boson production associated with jets
from QCD radiation and top quark production (see Fig.~\ref{f:wh}, left).

If the Higgs mass is higher than 135 GeV, its $WW$ decay becomes
dominant. In that case, both direct production
and associated production of a Higgs and an electroweak boson -yielding three
vector bosons in the final state- are promising search channels.

In direct $H \to WW$ searches, when both $W$ bosons decay to
an electron-neutrino or muon-neutrino pair the final state is quite clean,
with reducible backgrounds mostly due to Drell-Yan production of lepton pairs.
To discriminate direct production of a Higgs boson from non-resonant $WW$ production – which
in the standard model has a sizeable cross section~\cite{wwprod} —
it is useful to study the azimuthal angle $\Delta \Phi_{ll}$ between
the two charged leptons (Fig.~\ref{f:wh}, right),
since the zero spin of the Higgs boson and helicity
conservation conspire to produce leptons in the same direction in the transverse plane.

The sum of all known processes accounts nicely for the number of events
found in each search channel, and $95\%$ limits are set to the production cross section
of the Higgs boson as a function of its mass. Figure~\ref{f:limitsummary}
summarizes the present status of Higgs boson searches at the Tevatron.
It is necessary to note that most searches are still based on relatively small
amounts of data, while larger datasets still are being analyzed.
The standard model prediction for Higgs production appears still far away:
nevertheless, results are still roughly in line with what was predicted
by the 2003 Higgs sensitivity study~\cite{hswg}. That study foresees that by
the end of 2009 a light Higgs boson is likely to be discovered at the Tevatron,
or excluded for all masses below $135$ GeV. In order to reach that goal, a
combination of all results by CDF and D\O\ is mandatory. The two experiments
are collaborating well in these searches, and new limits will be produced as
more data is analyzed.

Top quark measurements

The large datasets of $p\bar{p}$ collisions
collected by CDF and D\O\ allow for many precision
measurements of top quark properties. Top quarks are particularly interesting
as laboratories of perturbative QCD, because of their large mass and short
lifetime. The most interesting measurement is however the one of the top mass,
which is a fundamental parameter of the standard model and has a large
impact through radiative corrections in the global fits to electroweak observables
attempting to verify the internal consistency of the model and predict the
unknown mass of the Higgs.

There are by now tens of determinations of $M_{top}$, using different
techniques and final states of top quark pair production. The single
most precise measurement has been obtained by CDF by reconstructing the top
mass in the single lepton final state of top pair production from
$940 pb^{-1}$ of collisions, resulting in 166 events containing a high-$P_T$ lepton,
missing transverse energy, and four jets, one of which originated by $b$-quark
hadronization.  A likelihood is
calculated for each event using the matrix element for leading order top pair production
and a parametrization of parton showering yielding the observed jets. The final measured
top mass is then extracted from a joint likelihood of the product of the individual
event likelihoods, where the jet energy scale uncertainty is convoluted with
the statistical error using an in-situ measurement of the hadronic $W$ boson mass.
The use of the $W$ mass as a calibration point allows to reduce the dominant source
of systematics, and the final measurement is $M_{top}=170.9 \pm 2.2 \pm 1.4 GeV$,
where the first error is the statistical plus jet energy scale uncertainty, and the
second accounts for all other systematics. Fig.~\ref{f:tevbest} (left) shows the reconstructed
$W$ and top quark masses in the sample.

That measurement, along with selected others, has been combined into a world average
of $M_{top}=171.4\pm2.1 GeV$, which is accurate to $1.2\%$ (see Fig.~\ref{f:tevbest}, right). Before the end of Run 2 the experiments are likely to get to a 1 GeV accuracy
on the top quark mass.

Electroweak physics

In the remainder of this paper it is only possible to mention one further recent
result on high-$P_T$ physics from the Tevatron experiments, namely the recent discovery
of production of pairs of $WZ$ bosons, a rare process of high
relevance for Higgs searches. CDF obtained 16 $WZ$ candidates by an optimized search for
triplets of charged leptons and missing transverse energy in $1.1 fb^{-1}$ of
data, where only 2.7 events were expected from background processes. Figure ~\ref{f:wzmet}
displays the distribution of the missing transverse energy in the events, showing
the clear signature of $WZ$ production at large missing $E_T$. Now one more process
of that kind is still missing, associated $ZZ$ production, and CDF observed one event
with the required characteristics, it will take some more data to measure that process
as well. The event display for the $ZZ$ candidate is show in Fig.~\ref{f:wzmet} (right).


The CDF and D\O\ experiments are producing remarkable physics results in high-$P_T$
physics measurements with the large datasets of $p\bar{p}$ collisions they have
collected so far during Run 2. If the Tevatron will provide the estimated luminosity
of 5-8 $fb^{-1}$ by the end of year 2009, there is a chance that the Tevatron will
beat the LHC in the quest for the Higgs boson. One less ambitious and more certain
target is reaching a precision in the top quark mass which will remain the most precise
measurement for many years to come. CDF and D\O\ look forwards to the last few years
of running with a lot of enthusiasm for the forthcoming challenges.


The author wishes to thank [This is where your name goes… ] for his/her editorial advice.


1. Helge - October 31, 2006

Again a few things.

1.) The first reference is not there in the postscript file (it shows [?]).

2.) It says D\O\ \dot Hadronic in the last paragraph of the first page. Looks like some kind of formating mistake… Maybe \dots?
There is also a backslash missing before a sqrt…
And a blank too much before the last dot.

3.) Another bad reference on the bottom of the last page.

Cheers, Helge

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