Transcript Search for the Standard Model Higgs boson with the CMS detector Tommaso Dorigo (INFN and University of Padova) for the CMS collaboration Introduction The.

Search for the Standard Model Higgs boson
with the CMS detector
Tommaso Dorigo (INFN and University of Padova) for the CMS collaboration
Introduction
The Standard Model (SM) Higgs boson can be
produced in proton-proton collisions at the LHC
by several different mechanisms (see Fig.1,
right). The production by gluon-gluon fusion
diagrams (see Fig.2, below) is the most
frequent, with cross sections of several tens of
picobarns; smaller is the production by vector
boson fusion (VBF), which however provides a
striking signature of two forward quark jets.
The two main production mechanisms have
been investigated to assess the chances of an
early detection of the Higgs boson with the
CMS detector. Three signatures are described
below:
1) ggHWW(*), with the decay of both W
bosons to en or mn pairs;
2) ggHZZ(*), with a subsequent decay of
both Z bosons to e+e- or m+m- pairs;
3) qqqqH, with a decay Htt accompanied
by two forward hadronic jets.
The Large Hadron Collider
The Large Hadron Collider (LHC) is a proton-proton synchrotron built 100 meters underground at the border
of Switzerland and France. It has a circumference of 27 km and is instrumented with 8-Tesla superconducting
magnets, providing the bending power to orbit 7 TeV protons in opposite directions.
Fig. 3: Schematic view of the LHC
complex. Four interaction regions along
Fig. 1: Higgs production cross section in proton-proton
collisions at 14 TeV c.m. energy, in femtobarns, as a
function of Higgs mass[1]. The green curve shows the cross
section of the gluon-fusion mechanism, the cyan curve the
cross section of Higgs production by vector boson fusion.
the beam are instrumented with particle
physics experiments: CMS and ATLAS,
which are optimized for high-energy pp
collisions, LHCb, which targets B hadrons
in forward physics processes, and a
nuclear physics experiment, ALICE.
Details of the CMS detector are provided
in a separate poster at this conference[2].
Protons in the beams travel in bunches which intersect inside the detectors at a frequency of 40 MHz. At full
power about 25 hard interactions per bunch crossing are expected, posing strong demands on the triggering
and data acquisition capability of the experiments.
LHC is due to start a 10-month data taking run in November 2009 at the reduced c.m. energy of 7 TeV, during
which less than one inverse femtobarn of collisions will be acquired by the experiments. The present study still
considers the Higgs search with CMS in a scenario of 1/fb collected at the c.m. energy of 14 TeV.
Fig. 2: Feynman diagram showing the mechanism of
direct Higgs boson production by gluon fusion at the LHC
The qqHqqtt search
The HWW search
The search for the WW(*) decay mode at CMS[3] employs events containing exactly two opposite-charge leptons (e or m) with transverse
momenta Pt>10 GeV and pseudorapidity |h|<2.5; at least one lepton must have Pt>20 GeV. The following additional pre-selection cuts are
then applied: a jet veto (NjetEt>15 GeV=0), large missing energy (Etmiss>30 GeV), and a dilepton mass above resonances (mll>12 GeV).
Two separate searches are performed: a cut-based analysis and a Neural-Network-based analysis (NN). In both cases the selection is
optimized using the azimuthal angle between the leptons, an upper cut on the dilepton mass, and requirements on lepton momenta and
missing energy. The NN analysis uses additional kinematic variables to separate the signal from the main backgrounds (see Fig. 4 below).
The analyses include complete techniques to determine background rates with control samples of data. The top pair background can be sized
up with events containing two additional jets, while the SM production of WW pairs can be normalized using data with mll>115 GeV.
The CLs method [1,2] is used to convert the number of expected signal and background events (Fig. 5) into a significance of the observable
signal, as a function of the Higgs mass. A first evidence for the Higgs boson is likely achievable in the region of best sensitivity (see Fig. 6).
Fig. 4: Output of the Neural
Network for signal (black points)
and backgrounds (histograms)
for a Higgs boson mass of 170
GeV. An optimized cut on the
NN output displayed here is
used to select the final
candidates.
Fig. 6: Expected
significance of the
excess of events that
can be obtained in the
Neural Network search,
as a function of the
Higgs boson mass.
Fig. 5: Number of events after the NN
selection, for different Higgs masses.
The HZZ search
In the HZZ(*) search[4] events are selected to contain four charged leptons
(e+e-e+e-, e+e-m+m-, or m+m-m+m-), with pair masses Mll>12 GeV.
To remove the Zb b and t t backgrounds further, CMS uses the combined
isolation of the two least-isolated leptons, and the significance of their impact
parameter with respect to the primary vertex. The reconstructed mass of the
dilepton pairs is requested to lay in the windows [50-100] and [20-100] GeV.
ZZ(*)
After the selection, backgrounds are almost exclusively due to SM
production. The four-body mass provides further discrimination, as shown in
Fig. 7. Mass window cuts allow to obtain a signal-to-noise ratio larger than one
throughout the 120-250 GeV Higgs mass region considered in the search.
The CLs method allows estimating the significance of the signal extractable
signal (see Fig. 8, right) with 1/fb of 14 TeV collision data. A sensitivity at the 2s
level can be obtained for favourable mass values.
References
[1] U. Aglietti et al., “Tevatron-for-LHC Report: Higgs”, arXiv:hep-ph/0612172,
December 17th, 2007.
[2] T.Dorigo, “Projected exclusion limits on the SM Higgs boson by combining
Higgs channels at the LHC”, poster presented at this conference, September 1st,
2009.
[3] CMS PAS HIG-08-006, “Search strategy for a Standard Model Higgs boson
decaying to two W bosons in the fully leptonic final state”, January 29th, 2009.
[4] CMS PAS HIG-08-003, “Search strategy for the Higgs boson in the ZZ(*)
decay channel with the CMS experiment”, February 7th, 2009.
[5] CMS PAS HIG-08-008, “Search for the SM Higgs boson produced in Vector
Boson Fusion and decaying into a t pair in CMS with 1/fb”, October 22nd, 2008.
A study of the vector boson fusion qqHqqtt
signature of Higgs decay, for Higgs masses between
115 and 145 GeV, has been performed[5] on events
containing one leptonic tau-decay candidate tenn or
tmnn, collected by a low-Pt electron or muon trigger.
A second tau-lepton candidate from H decay is
required to produce a narrow jet with Et>30 GeV and
one track with Pt>6 GeV within its core.
The two forward jets characteristic of VBF processes
provide a striking signature and are used to reduce
backgrounds, mainly coming from QCD multijet
production and Ztt decays. The mass of two Et>30
GeV forward jets has to exceed 400 GeV, and they
must be separated in pseudorapidity by more than 2.5
units.
After the kinematic selection backgrounds amount to
31.8 events, with 0.6-1.6 expected from the Higgs
signal, depending on Mh; a typical mass distribution is
shown in Fig. 9 below. Event counts are used to
determine expected limits on the signal cross section
in units of the predicted SM Higgs cross section, using
the CLs method (Fig. 10, bottom).
Fig. 7: Effective cross section as a
function of the four-lepton invariant
mass after the signal selection, in
the mixed eemm channel.
Four different signal distributions
(for masses of 130, 150, 200, and
250 GeV) are compared to residual
backgrounds. The latter are
essentially due to SM ZZ
production (in purple) and Zbb
production (in green).
Fig. 8: Values of the log-likelihood
ratio discriminant -2 Ln Q resulting
from the combination of the three
four-lepton final states studied in
the search.
The black curve shows the
expected distribution of the
discriminant in the absence of
signal as a function of Higgs mass,
while the red curve shows the likely
observable value of the likelihood
ratio in the presence of the Higgs
boson.
Fig.9: Reconstructed
mass of the two taulepton candidates in
1/fb of 14 TeV data after
the kinematic selection
described above. The
expected signal for
Mh=135 GeV (black
empty histogram) is
scaled up by a factor 10
for display purposes.
Fig.10: Expected limit
on the ppqqH Higgs
boson cross section
in units of the SM
value, as a function of
the Higgs mass. The
green and yellow
bands show 1- and 2standard deviation
bounds in the limit
that can be obtained.
Conclusions
The CMS experiment will search for a Standard Model Higgs boson in all possible final states. The HWW(*) and HZZ(*) decays provide
the cleanest signatures, and might allow to obtain a first evidence of the Higgs boson already with one inverse femtobarn of luminosity. For
the vector-boson-fusion channel qqHqqtt larger datasets are needed in order to reach sensitivity to Standard Model cross sections.
All studies presented here consider data-driven techniques to estimate the main backgrounds, and include in the calculation of significances
and limits a realistic scenario of achievable systematic uncertainties on background normalizations, Jet Energy Scale, lepton selection,
miscalibrations and misalignments.