Searches for New Physics
Many theories beyond the Standard Model predict new heavy resonance that can decay to top quarks, e.g. $Z'$ bosons.
A search for resonances decaying into top-quark pairs using fully hadronic decays has been performed at the PI
(JHEP 1301 (2013) 116).
By using the HTT to reconstruct two top quarks the di-top quark mass can be measured.
Distribution of the $t\bar{t}$ invariant mass $m_{t\bar{t}}$. The data, the Standard Model $t\bar{t}$ background prediction, the QCD multijet background prediction and a hypothetical $Z'$ signal with $m_{Z'} = 1$ TeV are shown. Data points show statistical uncertainties only. JHEP 1301 (2013) 116
The data are in agreement with the Standard Model prediction and limits on the mass and production cross-section of the hypothetical heavy particles have been set.
The top-quark is the heaviest particle in the Standard Model, as a
result of its
large coupling to the Higgs boson. Due to such a fundamental role
of the top-quark, many theories that could address existing questions
within the
Standard Model rely on new particles that also couple with the top quark.
In many of such models, such as models for quantum gravitation, one would
expect a new undiscovered particle to decay into a top-quark pair.
Our group searches for a particle decaying into top-quark pair, by comparing
the spectrum of the mass of the top-quark pair to a simulation, of what
one would expect to see if a new particle exists.
Reconstructed di-top quark mass as predicted in simulation and measured in data. ATLAS-CONF-2016-014
Buckets of Top and Higgs
In addition to the HTT (which of course can also be employed in measurements of Standard Model processes like the production of a
Higgs boson in association with a pair of top quarks ("$ttH$")) a second method to reconstruct pairs of hadronically decaying top
quarks is used by the ATLAS PI group. The "Buckets of Top" algorithm
(JHEP 1308 (2013) 086) aims to reconstruct top quarks
with transverse momenta $100 \mathrm{ GeV} < p_T < 400$ GeV, by sorting jets into three "buckets". The first two buckets $B_1$/$B_2$
will ideally contain the decay products of the two top quarks, while the third one $B_{ISR}$ collects the extra radiation in the event.
The performance and applicability of the method have been studied and method was validated in a Monte-Carlo to data comparison.
For the $ttH$ channel the bucket algorithm could be used to solve the combinatorial problem of assigning four b-quarks to the two
top decays and the $H\rightarrow b\bar{b}$ Higgs boson decay, as suggested in
JHEP 1402 (2014) 130.
Sketch of the buckets of top algorithm, $j_1$ to $j_{10}$ correspond to jets recontructed in the event.
Single Vector Like Quark search
The Standard Model has several open questions waiting to be answered,such as why
we have only six quarks and not more. The discovery of the Higgs boson excludes the existence of new quarks
similar to the ones we already have observed. However, a new class of quarks, called Vector
Like Quarks (VLQs) has not been excluded, as they do not receive their masses
through a coupling to the Higgs boson.
Leading-order Feynman diagram of single Q (Q=T,Y) production in Wb fusion and subsequent decay into Wb.
ATLAS-CONF-2016-072
Such Vector Like Quarks have not been observed, but they are a key component
in several Beyond the Standard Model theories, which address existing problems
of the Standard Model.
Our group works on the search for a new Vector Like Quark particle, which would
be produced in association with a b-quark and a light-quark, and further decays
in a W-boson and a b-quark. This search is performed in the channel on which the W boson decays into
an electron or muon.
Distributions of the reconstructed VLQ mass in simulation and data. ATLAS-CONF-2016-072