Department of Physics
College of Arts and Sciences

Experimental HEP Research
The Experimental High Energy Physics (HEP) group at Baylor is engaged in experimental elementary particle physics research at the European Center for Nuclear Research (CERN) in Geneva, Switzerland and Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, USA.

## Research at the CMS Experiment

The Baylor group officially joined the CMS experiment at the Large Hadron Collider (LHC) in May 2010, and since then we have already produced many significant physics results, and we are further enhancing our contributions to the CMS Collaboration. In 2012, the Baylor Tier-3 computing farm for CMS was commissioned, and this is strengthening our research activities in nearly all areas. The primary focus of our group in the CMS experiment has been a search for Supersymmetry (SUSY), but we also work on the measurement of jet internal structure. As our service contribution to CMS, we work on several projects on the hadron calorimeter (HCAL). Our group has also played a leading role in the missing transverse energy ($E_T$) working group.

Searches for Supersymmetry
Supersymmetry is one of the most well-known, yet unproven particle physics theories. The SUSY theory predicts a mirror particle for each elementary particle described by the Standard Model (SM) theory. The SUSY mirror particles would have identical quantum numbers as their SM counterparts, except for a difference in spin of a half unit. The SUSY partners of the SM bosons are then fermions (half-integer spin) and the SUSY partners of the SM fermions are then bosons (integer spin). While the SM particles have been observed, their SUSY partners currently have not - this implies that the SUSY particles have masses much heavier than their SM counterparts and need high energy collisions like those at the LHC for discovery. Confirmation of Supersymmetry would shed light on important questions in our current understanding of particle physics such as the gauge hierarchy problem, the unification of the electroweak and strong interactions, and the nature of dark matter. The Baylor group currently participates in two search channels: an inclusive SUSY search in the missing $E_T$ and multi-jet final state and a search for direct supersymmetric top quark production.
• Inclusive SUSY Search in the Missing $E_T$ and Multi-Jet Final State
An inclusive search in the missing $E_T$ and multijet final state is one of the most generic searches for SUSY. We search for proton-proton collision events that have large missing $E_T$ and at least three jets with high transverse momentum ($p_T$). The analysis is "inclusive" because no requirements are made on other high-$p_T$ particles such as leptons and photons. This search has a large expected acceptance of SUSY events at the cost of having to handle backgrounds from Standard Model events with great care. One of the main benefits of this inclusive approach is the potential to characterize the SUSY signal upon its discovery in an unbiased manner. Additionally, most of the robust analysis methodologies developed in this search can be applied to other searches in more exclusive final states with few modifications.

• Search for Direct Supersymmetric Top Quark Production
Beginning in 2012, we have pursued a dedicated search of $pp$ collision events for evidence of the production of supersymmetric (scalar) top quarks that result in a final state containing jets. After the discovery of the Higgs-like boson at 125 GeV/c2, "natural SUSY" has received more spotlight. The Higgs boson mass should have quadratically-divergent loop corrections due to a variety of SM particles, and heavy SM particles (e.g. the top quarks) have the largest contributions. In the naturalness arguments in which the supersymmetric partners of the SM particles cancel these loop corrections, the supersymmetric particles that should be rather light are the superpartners of the Higgs boson and top quark. The search for direct supersymmetric top quark (stop) production could well be the SUSY discovery channel.

Jet Internal Structure Measurements
When a quark or gluon is produced in a $pp$ collision, it undergoes hadronization, which is observed in a particle detector as a narrow, conical spray of particles. This spray of particles, called a jet, contains dozens of hadrons, which are particles composed of quarks. Measurements of the internal structure of jets give insights into the transition between partons produced in the hard $pp$ scattering process and the resulting hadron jets. Jet internal structure measurements are also sensitive to the effects of multiple parton interactions and the underlying event from beam remnants. These measurements are critical for tuning phenomenological models and simulations of particle collisions so that they provide a good description of jet fragmentation properties at the LHC energies.

The CMS hadron calorimeter (HCAL) is one of the key components of the CMS detector, especially for the physics analyses that the Baylor group is involved with. The hadron calorimeters measure the energy of hadrons that are typically contained inside of jets. Hadrons are composite particles made up of quarks that are bound together by gluons, and they interact with the material in the hadron calorimeter via the strong nuclear force.
• Baylor Contributions to HCAL Performance and Upgrade
The Baylor CMS group contributes to studying and improving the performance of the hadron calorimeter in three ways. First we are involved with the hadronic energy calibration, which is performed in order to derive response corrections and to establish a stable hadronic energy scale. Second, we help develop and use software for hadron data quality monitoring. The CMS hadron calorimeter can sometimes yield anomalous signals that were not from hadronic showers initiated by real particles in $pp$ collisions. We constantly examine hadron calorimeter data and either validate it or determine if their were such false signals. When problems are found, we work with the calibration team or reconstruction software team to resolve the issues. We contribute to the effort on characterizing anomalous signals and developing algorithms to remove such anomalous signals. Finally, Baylor team members are involved in the hadronic calorimeter upgrade project, which is necessary in order for CMS to fully profit from the enhanced LHC performance that is expected in the coming years. The goal of the upgrade is to address problems that have been uncovered during early operations.

Missing $E_T$ Reconstruction
The CMS detector is able to measure particles that interact electromagnetically or through the strong force, but some particles produced in a $pp$ collision, such as neutrinos, will not interact with the detector whatsoever. Their presence can instead be inferred due to conservation of momentum. Missing $E_T$ is the imbalance in the measured transverse momentum of observed particles in the detector. If a neutrino were produced in a collision, for example, it would not be detected. However, the presence of the neutrino would be inferred if there is any missing $E_T$ calculated; an estimate of the neutrino's transverse momentum would be the calculated missing $E_T$.
• Use of Missing $E_T$ in Physics Analyses
The reconstruction of missing $E_T$ is important for a wide variety of analyses at CMS that involve neutrinos in the final state, including SM measurements of the $W$ boson, top quarks, and tau leptons. Furthermore, numerous theories beyond the SM also predict the presence of particles that would carry momentum, but be invisible in the detector. For example, in the SUSY theory, large amounts of missing $E_T$ would arise due to the presence of the lightest supersymmetric partners, which are expected to by very weakly interacting. Accurate missing $E_T$ reconstruction is imperative for discovery of physics beyond the SM.

• Baylor Contributions to Missing $E_T$ Reconstruction
One of the major challenges for missing $E_T$ reconstruction comes from anomalous detector signals, non-collision background, and reconstruction failure that lead to a fake missing $E_T$ tail. Another big challenge for missing $E_T$ reconstruction has been the resolution degradation due to pileup interactions (multiple $pp$ collisions for a single event). In addition to monitoring the efficiency of missing $E_T$ reconstruction, the goal of CMS missing $E_T$ working group is to understand these challenges and establish techniques to mitigate and reduce these effects. The Baylor CMS contributes significantly to this effort. Furthermore, since the beginning of 2011, Dr. Hatakeyama has led the CMS missing $E_T$ working group.

## Research at the CDF Experiment

The CDF experiment collected $p\bar{p}$ collision data until September 30, 2011, the date of final collisions by the Tevatron accelerator at Fermilab. Baylor joined the CDF collaboration in 2003 and contributed significantly to successful detector operations and to several, diverse physics analyses. Our physics analysis efforts focused on three main areas: the search for the Higgs boson (in multiple channels), measurements of single top quark production, and a model-independent search for photon + jet + missing $E_T$ events. Before the final Tevatron collisions, Baylor also provided service work to the CDF experiment by providing manpower for data taking shifts. The group was furthermore responsible for the L1 tracking trigger, known as the XFT for eXtremely Fast Tracking.

Higgs Searches
The origin of mass is still a mysterious question in the SM theory. Our current understanding suggests that mass is generated via the Higgs mechanism through a process called spontaneous symmetry breaking. The SM theory predicts that the Higgs mechanism would give mass to elementary particles and would furthermore produce a spin-0 particle, called the Higgs boson. Discovery of this particle would provide direct evidence for the Higgs mechanism and an explanation for the origin of mass. Significant work was performed in search of this particle at the CDF experiment.

In December 2006, Baylor joined the Higgs Discovery Group at CDF. Since that time, members of our group participated in a combined effort to optimize all aspects of the Higgs search at the Tevatron. We searched for the SM Higgs boson produced alongside a $W$ boson, for a SM Higgs boson decaying to a pair of photons, and for a beyond-the-Standard-Model Higgs boson decaying to a pair of photons. In addition, one group member was a member of the Tevatron New Phenomena & Higgs Working Group (TEVNPHWG), and he worked directly on the calculation of the combined CDF and Tevatron Higgs limits. In light of the discovery of a new boson at the LHC announced in July 2012, the activities of the CDF Higgs Discovery Group and TEVNPHWG have continued to be extremely relevant and influential.
• Associated Production of a SM Higgs with a $W$ Boson
While the most sensitive searches for the SM Higgs boson at the LHC are those based on its decays into pairs of gauge bosons ($H\rightarrow\gamma\gamma$ and $H \rightarrow ZZ$), searches based on decays into pairs of $b$ quarks were the most sensitive at the Tevatron. A Higgs boson produced with a $W$ boson (called the $WH$ channel) was among the most sensitive processes for searching for a Higgs boson in the mass range between about 110 and 150 GeV/c2. At the Tevatron, the relatively large production rate for the $WH$ process, combined with the dominant decay of the Higgs to $b\bar{b}$, made $WH \rightarrow l \nu + b\bar{b}$ an ideal search channel. Although the Higgs boson production rate when produced from two gluons is about ten times larger than that of $WH$, the $gg \rightarrow H \rightarrow b\bar{b}$ channel is not viable due to the large $b\bar{b}$ background from SM QCD processes. In the $WH$ channel, the presence of a lepton from the decay of the $W$ boson reduces the huge $b\bar{b}$ background rate at the Tevatron.

The results from all search modes involving decays into pairs of $b$ quarks at CDF, combined with similar results from the D0 experiment, led to evidence for a new particle in the area of 125 GeV/c2 that is consistent with the new boson observed at the LHC experiment. Although the excess does not bear the same statistical significance as that of the LHC measurements, the Tevatron $b\bar{b}$ search results are unique and have resulted in multiple publications. The combined results from the CDF and D0 data are described in a Phys. Rev. Lett. publication from 2012.

• SM Higgs Decaying to Two Photons
Although low-mass Higgs boson searches at the Tevatron usually focused on the dominant $b\bar{b}$ decay channel, the $\gamma\gamma$ final state was also an appealing search mode despite its small branching fraction, which peaks at 0.23% for a Higgs boson mass of 125 GeV/c2. For this channel, the Higgs can be produced via the fusion of two gluons ($gg\rightarrow H$), which has a production rate much larger than WH (thus producing more Higgs bosons). The $H\rightarrow\gamma\gamma$ channel was also appealing due to the clean profile ("signature") the photon pair leaves in particle detectors. While b quarks will fragment to produce jets in the detector, which are difficult to fully reconstruct, photons deposit an isolated cluster of energy. This allows for a larger fraction of $H\rightarrow\gamma\gamma$ events to be reconstructed compared to the $H\rightarrow b\bar{b}$ process. The clean photon signature also leads to a narrow diphoton mass peak in the data, which is a powerful discriminant against smoothly falling diphoton backgrounds from other SM processes with two photons (or photon-like objects). It is these experimental features that have helped the ATLAS and CMS experiments at CERN discover a new Higgs-like particle near a mass of 125 GeV/c2 using the diphoton final state.

Though the sensitivity for observing $H\rightarrow\gamma\gamma$ signal was smaller at the Tevatron than at the LHC, the results obtained by CDF were important in the overall CDF and Tevatron Higgs searches. The CDF $H\rightarrow\gamma\gamma$ results were combined with other Higgs boson searches at CDF and D0 in order to gain as much sensitivity as possible to a Higgs boson observation at the Tevatron. We furthermore developed enhanced search techniques to optimize a search for $H\rightarrow\gamma\gamma$, which have been described in two publications, one of which uses the full diphoton data from CDF (10.0 fb-1).

• Fermiophobic Higgs Boson Decaying to Two Photons
In the SM, the Higgs mechanism that endows elementary particles with mass requires a single doublet of complex scalar fields. But does nature follow this minimal version or does it require a multi-Higgs sector? Some beyond-the-Standard-Model theories predict extended Higgs sectors with additional doublet/triplet fields. One such model incorporates two doublets of complex scalar fields to generate five scalar Higgs bosons, three of which are neutral. For type-I models, and for certain choices of the model parameters, one of these neutral scalars couples only to the fermions (therefore giving them mass) and another couples exclusively to the bosons (therefore giving them mass). The latter Higgs scaler is referred to as the "fermiophobic" Higgs boson. Fermiophobia can only arise in certain Higgs models, and in particular is not predicted by the minimal supersymmetric model (MSSM). Therefore, the observation of a fermiophobic Higgs would be an evidence against MSSM.

The implementation of the Higgs mechanism in the SM may not be the one that nature has chosen. Therefore, the group searched for evidence of a Higgs boson in this alternative fermiophobic Higgs model, which predicts substantially higher $H\rightarrow\gamma\gamma$ branching ratios than that of the SM. In addition to optimizing the analysis for fermiophobic Higgs boson production, techniques developed by the SM $H\rightarrow\gamma\gamma$ analysis at CDF were applied. This is described more in a Phys. Lett. B journal article. The full CDF diphoton data (10 fb-1) excludes the possibility of fermiophobic Higgs masses below 114 GeV/c2 at the 95% CL.

Single Top Quark Search
In the Standard Model, top quarks are typically produced via the strong interaction such that a top and antitop pair ($t\bar{t}$) are produced together. Mediated by the $W$ boson though the electroweak interaction, the SM also allows the production of a single top quark alongside multiple jets. The production of single top quark events was first observed by the CDF and D0 experiments at the Tevatron in 2009, about 15 years after the discovery of the top quark in $t\bar{t}$ events. Since then, Baylor CDF members made extensive and crucial contributions to CDF's single top analyses by adopting a next-to-leading order Monte Carlo generator to provide precise modeling of single top production and optimizing a neural network training process to better constrain systematic uncertainties on the measured single top quark production rate.
• Motivation
The reasons for studying single top production are compelling. In electroweak interactions, quark flavor changing is possible through charged currents by the exchange of a $W$ boson. The probability of transition from one flavor of quark to another flavor of quark is described by the Cabibbo–Kobayashi–Maskawa (CKM) matrix, the values of which are determined from experiment. The matrix element $V_{tb}$ governs the decay of the top quark, which typically involves a transition to a bottom quark. The single top production rate is directly proportional to the square of |$V_{tb}$|, and thus a measurement of the rate provides a unique opportunity for a direct measure of the $V_{tb}$ element. A measure of this element constrains fourth-generation models, models with flavor-changing neutral currents, and other new phenomena. Second, it would be extremely valuable and interesting to search for a CP-violating asymmetry in single $t$ vs. single $\bar{t}$ production. Since the top quark decays before hadronization, its polarization can be directly observed in the angular correlations of its decay products. The single top quark events provide a nearly pure polarization sample, which allows a probe of the spin projection of the top quark and the chirality of the $W$ boson. Finally, a precise measurement of the single top cross section for one production mode (the t-channel) can help constrain the high-x gluon parton distribution function, which would greatly help to reduce the uncertainties associated with theoretical predictions for the LHC.

Model Independent Search
There are numerous new theory models attempting to address shortcomings of the standard model of particle physics. Many of these models, including SUSY, predict mechanisms that could produce a photon + jets + missing $E_T$ signature. The group completed a model-independent signature-based search for anomalous events in this channel by scanning kinematic distributions for an excess of events over SM predictions. These kinematic distributions included missing $E_T$, invariant mass of the photon + leading jets, and total transverse energy ($H_T$). An excess could indicate the existence of a new heavy particle decaying into photon + jets or a new physics mechanism such as gauge-mediated SUSY breaking.
• Shortcomings of the SM
The SM has been tested extensively by experimental data. So far, no evidence contradicting SM predictions has been observed. Yet, there are many questions that are not addressed or verified by the SM.
• It provides no prediction of the masses of the fundamental particles: the quarks and leptons.
• It does not explain why there are only three families of leptons and quarks.
• It does not explain why gravity is so weak, and it is not able to describe its effects at the quantum level.
• The neutrinos were assumed to be massless. Yet recent experimental results show evidence of neutrino oscillations, which indicate that neutrinos may have a tiny but non-zero mass.
• There is no natural candidate for dark matter. Cosmology and astronomy suggests that 70% of the universe is dark energy and another 25% is made of dark matter, meaning only 5% of visible matter is explained by the SM.
• Analysis Results
In this analysis, we found good agreement with Standard Model predictions extending over several orders of magnitude. The search for new heavy particles in high-$E_T$ events has shown no significant deviation from data. Furthermore, the search for a narrow resonance has shown no indication of the presence of heavy particles decaying to $\gamma$ + jets. We conclude that all of our measurements are in agreement with Standard Model expectations.

eXtremely Fast Tracker
The huge amount of $p\bar{p}$ collision data produced at the Fermilab Tevatron posed a big challenge for data processing and storage. The Tevatron delivered 1.7 million $p\bar{p}$ collisions per second (1.7 MHz), however CDF could keep only about 100 Hz of data in storage media for offline analysis. While the background rejection was important, a high and unbiased efficiency for signals was a necessity. A trigger system was incorporated, which made a decision in real time as to whether a $p\bar{p}$ collision event was interesting or not. Triggered events that passed a set of selection requirement would be stored on disk for later processing.
• Baylor Contribution to the CDF Trigger
The CDF trigger and data acquisition system had three levels of event selections. The Baylor HEP group played a significant role in the design, installation, commissioning, operation, and maintenance of the eXtremely Fast Tracker (XFT), the Level 1 track trigger. If a particle is charged, it will interact with the tracking chambers of the detector, leaving hits along its path. These hits were used to determine the charged particles trajectory, called a track. The purpose of the XFT was to reconstruct in real time the path of charged particles in the outer regions of the detector. The performance of the XFT was excellent over the years, functioning better than the design specification at low instantaneous luminosity. As the performance of the Tevatron improved over time, an upgrade to the original XFT system was needed to accommodate the high-luminosity running environment. The Run 2B XFT upgrade was completed for this purpose. Baylor was one of six university groups that participated on the Run 2B XFT upgrade project, and provided the primary operational and maintenance support for the entire system.