The ATLAS group at
Northern Illinois University
is one of the two general-purpose detectors getting ready to study
proton-proton collisions at a center-of-mass energy of up to 14 TeV to be
delivered by the
Large Hadron Collider
Several members of the
Experimental High Energy Physics Group at NIU
participate in the ATLAS project.
The ATLAS group at NIU currently consists of
- Dr. Gerald Blazey (professor,
on leave of absence from 2011-06-01 to 2013-05-31),
- Blake Burghgrave (graduate student),
- Rob Calkins (graduate student),
- Stephen Cole (graduate student),
- Dr. Dhiman Chakraborty
(professor, team leader),
- Dr. Jose Guilherme Lima
- Dr. Stephen Martin,
- Chad Suhr
- Dr. Sergey Uzunyan
- Dr. Adam Yurkewicz
- Dr. Vishnu Zutshi
(senior research scientist).
The NIU group is engaged in several critical tasks in operations of the ATLAS
detector, its planned upgrade, and high-priority physics analyses.
Much of these activities are conducted in close collaboration with
the Argonne National Laboratory.
The NIU group is contributing to the running of the ATLAS experiment through
the following tasks (* = current):
In the past the team has contributed to
- Operation of the scintillator-tile hadronic calorimeter (TileCal):
- Online Data Quality Monitoring (DQM) and signal processing [Suhr];
- Offline DQM and calibration, coordination [Calkins, Cole*].
- Data preparation for offline DQM [Cole*].
- TileCal software:
- Simulation, documentation, co-ordination [Lima*].
In addition to offline development, maintenance, and support activities, team
members also take detector and software operation shifts appropriate for their
areas of involvement.
- Detector simulation:
- Interaction of particles in the detector volume, using GEANT4 [Lima];
- Translation of the result of the previous step into recorded data,
a.k.a. "digitization" [Chakraborty];
- Digitization simulation for the Tile calorimeter [Zutshi].
We are participating in the Fast Track Trigger (FTK) project to improve
ATLAS' track-based triggering capabilities in the Phase 1 upgrade [Blazey, Burghgrave, Chakraborty, Yurkewicz].
Beyond that, a Phase 2 upgrade aims for a ten-fold increase in the
average instantaneous luminosity.
Extensive modifications to many parts of the detectors will be necessary to
perform on par or better than the initial implementation despite the increased
We have contributed to the simulation and algorithm development for
the forward pixel tracker (disks) [Lima].
The NIU team's main physics interests are in searches for physics beyond the
Standard Model in production and decay of top quarks and studies of
triple gauge coupling through WZ production at the LHC.
- Single top production -
In the standard model (SM), top quarks can be produced singly in
proton-proton collisions only through charged-current electroweak
In the presence of an alternative mechanism, through charged Higgs bosons,
for example, the total production rate, the difference between the rates
of top and antitop production, as well as the kinematics of the final state
would be different from that predicted by the SM.
We propose to perform a simultaneous comparison (fitting) of all these
quantities between SM predictions and ATLAS data and interpret the result
in the light of new physics beyond the SM.
This is the subject of Chad Suhr's Ph.D. thesis (advisor: Chakraborty).
- Top-antitop pair production and decay -
In the SM, top-antitop pairs are produced in hadron collisions
(i.e. proton-proton at the CERN's LHC or proton-antiproton at Fermilab's
Tevatron) almost entirely through strong interactions.
Alternative mechanisms are predicted by a number of different extensions
of the SM.
Most of these involve a neutral heavy intermediate vector boson,
generically referred to as Z', although scalar intermediaries appear
in some models as well.
The presence of such alternatives are expected to alter production rates
and kinematic distributions with respect to SM predictions.
On the decay side, the SM predicts that a top quark should decay
essentially always to a W boson and a b quark.
However, some theories, including supersymmetry (SUSY) contain a richer
Higgs sector than the SM.
In such theories where a charged Higgs boson appear, it can serve as an
alternative to the W boson in both the production and decays of the top
However, couplings of the charged Higgs being very different from those of
the W, the branching fractions into various final states would be
Furthermore, the yet unknown mass of the charged Higgs (if it exists),
are expected to alter the kinematic characteristics of the final states.
Investigation of such possibilities based on simultaneous (global) fitting
of lepton+jets and dilepton final states is the subject of Rob Calkins'
Ph.D. thesis (advisor: Chakraborty).
- Top-antitop pair decays into tau+jets final state -
If the charged Higgs boson is lighter than the top quark, then t->bH+ is
If the parameter tan(beta) is large (> 1), then the charged Higgs boson
should decay predominantly into a tau lepton and a neutrino.
In that case, the most direct appearance of the charged Higgs should
be evident in an excess of events containing tau leptons.
Members of the NIU HEP group have been long involved in studies of
ttbar -> tau+jets at the Tevatron (D0 experiment).
Lima, Chakraborty, and Burghgrave (advisor: Chakraborty) are contributing
to similar studies at ATLAS.
- WZ production -
Pair-production of weak bosons is the most direct way to study their
While the gauge boson masses and their couplings to fermions have been
tested extensively at LEP2 and Tevatron, the triple gauge boson coupling
have not been determined with the same level of precision.
Since the triple gauge coupling are uniquely determined by the gauge
structure of the Standard Model, a thorough examination of diboson
production at the LHC is key to testing the high energy behavior of the
electroweak interactions and to serve as a probe of new physics in the
vector boson sector.
WZ production followed by decays to fully leptonic final states
(e, mu, nu) gives the best signal-to-noise ratio.
The rate is predicted to be very small in the SM, but can be significantly
enhanced in scenarios beyond the SM.
Zutshi and Cole (advisor: Blazey) are engaged in these studies.
- Searches for supersymmetry -
Especially models with a mass spectrum that is compressed compared to
Theorist/phenomenologist Dr. Martin collaborates with experimenters at NIU
and ANL on this.
In relation to and extension of the above interests and activities, members
of the group contribute, or have contributed, to several other tasks
- Calibration of b-tagging using the ttbar candidate pool [Calkins],
- Optimization of W+(heavy)jets Monte Carlo generator parameters [Calkins],
- Detector performance studies (identification of photons and electrons,
b-jets) [Calkins, Zutshi],
- Monte Carlo production and validation [Suhr],
- Calibration of forward jets under pile up [Suhr],
- Tau+jets trigger studies [Lima],
- Service on a number of editorial boards [Blazey, Chakraborty, Zutshi],
- Co-convenership of US-ATLAS top physics forum [Chakraborty],
- Coordination of the US-ATLAS Analysis Support Center (ASC) at ANL [Blazey].
The NIU ATLAS group's research is supported by grants from the National Science Foundation and the
U.S. Department of Energy.
For more information, contact
Prof. Dhiman Chakraborty.
Last updated: DC, 2012-07-24.