Dr. Bhawna Gomber

Research Summary

My research interests lies in understanding the mystery of dark matter. My thesis work involved search for anomalous production of mono-photon events along with momentum imbalance in transverse plane. If an excess is found, could be interpreted as dark matter recoiling from the high PT photons. After my Ph.D degree, I spent rst year of my post-doctoral term working on \The LUX-Zeplin (LZ)" liquid-xenon WIMP dark matter direct search experiment, understanding the background techniques and simulations. Since the beginning of LHC run II in November 2015, I am stationed at CERN and continuing with dark matter search with run II data of the CMS experiment. Since both the direct-detection and collider searches compliment each other, I have the experience to understand the technicalities on both fronts.

Understanding of detector and software of both "CMS" and "LZ" experiments.
My research activities at CMS includes analyzing data for the search of: Dark Matter particles, Extra Dimensions, Trilinear Gauge Couplings and Invisible Higgs decays.
My research activities at LZ includes understanding the radiogenic backgrounds, validation of the LZ simulation using Noble Element Simulation Technique (NEST) model.
I have been working as the trigger expert for exotica search analysis team of the CMS experiment.
During my research appointments I have guided and worked with many Ph.D and masters students from different universities.
I have also organized tutorials for CMS Data Analysis School at Fermilab.
I have presented several talks and posters in many international conferences based on my work and on behalf of both the CMS and LZ collaboration.

Research Experience during Ph.D

Mono-Photon (Photon + Missing Transverse Energy (MET)) Analysis

During my Ph.D I worked on search for anomalous production of mono-photon events recoiling from missing transverse energy with the CMS detector at 8 TeV. Photon+MET is a canonical signature to con rm the presence of dark matter candidates at colliders which can be detected as highly energetic photon and weakly interacting dark matter candidate which escapes the detector leaving a huge missing transverse energy. This nal state also has sensitivity to models of extra spatial dimensions, in particular Arkani-Hamed, Dimopoulos,and Dvali (ADD) model which provides a possible solution to hierarchy problem. I worked on all aspects of the analysis including data/MC scale factors to account for the difference in efficiencies between data and MC predictions, effect of analysis cuts against pile-up, background estimation using Monte Carlo simulation, background dominated control regions to validate the analysis cuts and calculation of efficiencies, acceptances and systematic uncertainties. Also estimated electron and jets faking a photon background using the data-driven techniques. In summary, an upper limit of 14.0 fb on the cross section is set at the 95% confidence level for events with a monophoton final state with photon transverse momentum greater than 145GeV and missing transverse momentum greater than 140GeV. Constraints are set on production and translated into upper limits on vector and axial-vector contributions to the X-nucleon scattering cross section, assuming the validity of the Effective field theory framework (EFT). For M =10GeV, the x-nucleon cross section is constrained to be less than 2:6 10􀀀39cm2(9:6 x 10-41cm2) for a spin-independent (spin-dependent) interaction at 90% confidence level. These limits compliments the results from the direct-detection experiments. For ADD extra dimensions, gravitational energy scale up to 2.12 - 1.97 TeV, depending on the number of extra dimensions, are excluded.

I was the analysis contact and editor of internal analysis note and paper.Results are public in the form of PAS (EXO-12-047) and published in Phys. Lett. B 755 (2016) 102. I have also participated in the 7 TeV results of the same analysis which got published in Phys. Rev. Lett. 108, 261803 (2012).

Measurement of Z( )gamma production cross section and limits on anomalous triple gauge couplings (aTGC) at 8 TeV

The production of a pair of bosons in particle collisions provide an opportunity to test the electroweak sector of the SM. The couplings of these boson interactions are well known in SM and any deviation in production rate will be an indication of new physics and anomalous triple gauge coupling (aTGC). In this analysis I have studied the production of Z where Z decays invisibly as it is sensitive to the ZZ and Z anomalous coupling. I was the analysis contact and editor of internal analysis note and paper. Results at 8 TeV are published in Phys. Lett. B 760 (2016) 448 and were the most sensitive results in the Z final state.

Measurement of Wgamma and Zgamma production cross section at 7 TeV In this analysis,

I have studied the performance of muon triggers and overall muon re-construction criteria using the tag and probe method for the Zgamma and Wgamma in the muon nal state. I have worked on combining the Wgamma and Zgamma measured cross-section in electron and muon nal state at 7 TeV using the Best Linear Unbiased Estimator (BLUE) method.

Validation of HCAL Simulation Hits

CMS software release keeps on changing with time due to modifications in reconstruction and to implement the detector understanding into the software which requires the validation of CMS software. Involved in writing package of Simulation Hits in HCAL which is included in central validation sequences. This package includes the information of cumulative hits in terms of the energy, time, energy-weighted time, occupancy in ietaiphi plane of the hits in different time window for all the sub-detectors of HCAL in their corresponding depths. Also extended the code for the upgrade geometry.

HCAL Forward Backend Electronics Upgrade

After the Long Shutdown 1 (LS1), current VERSABus Memory card(VME) based systems for the data taking will be replaced by microTCA(uTCA) which will lead to change in the electron ics. For LS1 only backend electronics of the HCAL Forward(HF) will change. SINP is responsible for making the uHTR cards in collaboration with University of Minnesota. I am participating in the project from the beginning and my main contribution had been in setting up a lab at the institute for testing of the uTCA crate and different components of the uHTR cards. We tested the uTCA crate for the basic functionality i.e powering on/o through telnet, setting the crate ID, programmed the front and back FPGA using ATM EL tools and checked the stability of the power mezzanines of the uHTR cards for 40 hrs in four test cycles (up, down, normal, high) which provides power to the electronic components of the uHTR card. Testing of the power mezzanines requires different setup and now I am helping other institutes in India with this setup. Also performed the Bit Error Rat e(IBER) tests by sending the psuedo data from optical transmitter to receiver and con rmed the error free transmission in 1011 bits. I have extensively contributed in thes e tests both at 904 and SINP.

During 2015 a proposal is to operate VME and uTCA readouts for HBHE in parallel, which requires the splitting of each of the frontend data bres. The 3.5dB loss in the splitters imposes strict requirements on the sensitivity of the optical receivers which requires studying the sensitivity requirements for the receivers. I worked with the Maryland team in the measurements of the Optical Power and Optical Modulation Amplitude (OMA) delivered by all of the 1656 active HBHE bres and also measured the sensitivity of the VME and uTCA optical receivers by attenuating the 1.6 Gbps signals from VECSELs until BER reaches 10􀀀10. We confirmed the power budget i s adequate for the VME optical receivers and marginal for the uTCA optical receivers.

Research Experience after Ph.D

Research Experience at LZ

Background Studies at LZ

Understanding radiogenic and other backgrounds is crucial to the success of LZ. The LZ background modeling program is based on Monte Carlo simulation of and neutron generation and propagation throughout the detector and its surrounds, with the internal source strengths based on screening results and in situ measurements. The PMTs contribute a significant level of background. I performed first background study for so-called gamma-X events in which a gamma ray originates in the material of a bottom PMT, Compton scatters in the reverse field region of the xenon in which the electrons are not collected, producing only an S1 signal, and scatters again promptly in the forward field region of the TPC resulting in a composite S1 signal. Electrons in the second scatter are collected and produce a later S2 signal small compared to that expected for a single Compton scatter so more likely to fake a nuclear recoil signal.

This background study was an exercise in using the preliminary LZsim simulation based on LUXSim with preliminary TPC grid, field shaping, and PMT geometry models. From these results and the expected PMT material activity levels, the gamma-X background event expectation in 300 live days is preliminarily calculated to be 0.45 events for an energy window between 1.5 and 6.5 keVee. These studies were also used in the LZ Conceptual Design Report.

Validation of Noble Element Simulation Technique (NEST) program

The LZ simulation uses a variety of nuclear interaction and decay codes and low energy particle transport codes in a customized GEANT framework. I worked in validating the Noble Element Simulation Technique (NEST) model with LZ collaborators including Mathew Szydagis. The NEST code is at the core of the simulation of the scintillation light called S1, a signal sensitive to among other things the electric field strength.

Computing and Simulations

I worked with other Wisconsin members, T.Sarangi, S.Dasu and C.Vuosalo to setup a portal for the LZ collaboration for the Open Science Grid. The LZ virtual organization (VO) is now working enabling collaborators from across the collaboration to use both computational and storage resource in a location independent way. Opportunistic computing cycles amounting to thousands of hours per day are available across OSG and UK-GridPP for LZ as a result of this work. Using the CVMFS server, I helped our team at UW to initialize the distribution of existing LZ software, such as MC simulation code, LUXSim, special CLHEP and Geant4 libraries used in LUXSim and ROOT framework. Using these computing facilities, I simulated different sources to study the gamma-X backgrounds.

Research Experience at CMS

To explore the nature of dark matter, I initiated a number of dark matter searches at Wisconsin, which look for large missing transverse energy produced in association with neutral electro-weak vector bosons such as photons and Z bosons. If the dark matter is made of particles, which are light enough and interact with sufficient strength with normal matter, the LHC should be able to produce them in proton-proton interactions

Mono-Photon Analysis

During my post-doctoral period, I have extended this search working with graduate student J. Buchanan and T.Perry (UW) analyzing the 13 TeV data in both 2015 and early 2016 LHC data. The results are consistent with SM production rates resulting in limits in the context of simplified models, which are also converted, with certain assumptions for mediator mass and spin-parity, to dark matter nucleon interaction cross section limits to compare with direct dark matter search experiments such as LUX. The results are also interpreted in the context of models with extra dimensions resulting in a 95% CL lower limit on the modi ed Planck scale, MD. Results are public in the form of PAS (CMS PAS EXO-16-039, CMS PAS EXO-16-014) and the 2016 early data results are going for publication.

Mono-Z Analysis

I have worked with graduate student N. Smith (UW) to search for a Z-boson, reconstructed using its ee and uu decays, recoiling from the missing momentum using both 2015 and 2016 data. The results are consistent with the expected background from the Standard Model processes, dominated by the Z(ll)Z( ) process at high ET miss. This result enabled placing exclusion limits on simplified dark matter models, and is interpreted as a 95% CL upper limit on the dark matter-nucleon interaction cross section, for vector and axial-vector coupling. Results are public in the form of PAS (CMS-PAS-EXO-16-010, CMS PAS EXO-16-038) and are going for publication with 2015 results.

Mono-H Analysis

I have recently begun a mono-higgs analysis with graduate student L.Dodd (UW), to search for H( ) candidate recoiling from large missing transverse energy. The connection of the Higgs boson with the dark matter is an especially important case to study. In Higgs portal models, an additional Z' boson exists and decays primarily to the SM-like Higgs boson (h of 125 GeV) and a heavy Higgs bosons (e.g., the heavy pseudo-scalar A in 2HDM). The A in turn decays primarily to a pair of weakly interacting massive particles, which are dark matter candidates. Initial results obtained by our group using the -pair channel are promising and compare favorably to the b-pair channel recently reported at ICHEP.

Mono-Z0 Analysis

I have recently begun a mono-Z0 analysis with graduate student U.Hussain (UW), to search for a dark matter candidate interacting with the standard model fermions through a very weakly coupling light vector boson (Z0). In this model, pioneered by Prof. Yang Bai (UW) and collaborators, a thin pencil-like jet recoiling from a large missing transverse energy is produced. The pencil jet finder and analysis framework is being setup. Early results are quite promising.

Invisible Higgs Decay

There is a large range of parameter space in several new physics models, e.g., MSSM, where there exists a H(125) that can decay invisibly. Current accuracy of the LHC datasets allow signi cant room, amounting to a 35% branching ratio of H(125) to invisible states. A search for such decays is also motivated by the possibility of a discovery of a dark matter particle candidate through this Higgs portal. I have worked with graduate student N. Smith (UW) to search for invisible Higgs decays using ZH associated production mode, where the Z-boson decays to light leptons. No signal has been observed, resulting in our setting an upper limit on the production times branching fraction of H(125), which is produced in association with a Z boson and subsequently decays invisibly. In combination with the VBF-tagged and ggH tagged analyses, we limit the invisible branching fraction of H(125) to below 20%. Results are public in the form of PAS (CMS-PAS-HIG-16-008, CMS PAS HIG-16-016) are going for publication with 2015 results.

CMS-ATLAS DM Forum

I have contributed in the LHC CMS-ATLAS Joint DM forum in validating the recommendations for the simplified models that is being used extensively in both the collaborations.

Measurement of Z( )gamma production cross section at 13 TeV

The production of a pair of bosons in particle collisions provide an opportunity to test the electroweak sector of the SM. The couplings of these boson interactions are well known anomalous triple gauge coupling (aTGC). In this analysis we have studied the production of Z as it is sensitive to the ZZ and Z anomalous coupling. Results at 13 TeV are public in the form of PAS (CMS PAS SMP-16-004).

Calorimeter trigger operations The development of the Level1 calorimeter trigger(L1CT) monitor, control, and emulation software is a UW responsibility. I worked on developing the emulator (which duplicates the exact bit-by-bit function of the hardware) and maintaining it and checking system performance with physics data, as well as using it to update the L1CT calibration and improve the calorimeter trigger performance.