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The CMS Collaboration investigated the production mechanism of t$\overline{\mathrm{t}}$ pair production in a search for a charged Higgs boson decaying to charm and bottom quarks (H$^+\to$c$\overline{\mathrm{b}}$) using 19.7 fb$^{-1}$ of proton-proton collision data at $\sqrt{s} =$ 8 TeV. The study focused on identifying events with at least four jets, a charged lepton (muon or electron), and missing transverse momentum as signatures of the decay process. A kinematic fit was performed to distinguish between jets likely originating from direct top quark decays and those potentially associated with the decay of the charged Higgs boson. However, no evidence for the presence of a charged Higgs boson was found in the data. Upper limits at 95% confidence level were set on the branching fraction $\mathcal{B}$(t $\to$ H$^+$b) in the range of 0.8-0.5%, assuming $\mathcal{B}$(H$^+\to$c$\overline{\mathrm{b}}) =$1.0 and conservation of total branching fractions for top quark decays. These limits were established for charged Higgs boson masses ranging from 90 GeV to 150 GeV. Figure 2 illustrates post-fit dijet mass distributions from standard model backgrounds compared with expected signal yields for different scenarios involving varying numbers of b-tagged jets and different charged Higgs boson masses (MH+). This analysis provides valuable insights into the search for exotic particles in high-energy collisions at CERN's Large Hadron Collider facility.
SummaryScientists at CERN studied how special particles called top quarks and their partners are made. They looked for a new particle called a charged Higgs boson that breaks apart into charm and bottom quarks. They used lots of data from smashing tiny particles together to do this study. The scientists searched for specific kinds of events with certain numbers of jets, a type of particle, and missing energy. They tried different methods to tell the difference between jets coming from different types of particles but didn't find the charged Higgs boson they were looking for.
Definitions
- CMS Collaboration: A group of scientists working together on experiments at CERN.
- tt̅ pair production: Making pairs of top quarks and their antiparticles.
- Higgs boson: A fundamental particle associated with giving other particles mass.
- Proton-proton collision data: Information collected from when protons smash into each other at high speeds.
- Jets: Groups of particles produced in high-energy collisions.
- Charged lepton: An electrically charged version of a light elementary particle like an electron or muon.
- Kinematic fit: A method used to match experimental data with theoretical predictions in physics experiments.
- Branching fraction: The probability that a specific decay process will occur in particle physics.
- Upper limits: The maximum values allowed for certain measurements in scientific studies.
- GeV (Giga-electronvolt): A unit used to measure energy in particle physics, equal to one billion electronvol
Introduction
The search for new particles and phenomena is one of the main goals of high-energy physics. In this pursuit, the CMS Collaboration at CERN's Large Hadron Collider (LHC) has been conducting various studies to understand the fundamental building blocks of our universe. One such study focuses on investigating the production mechanism of t$\overline{\mathrm{t}}$ pair production in a search for a charged Higgs boson decaying to charm and bottom quarks.
This research paper, titled "Search for Charged Higgs Bosons Produced via Vector Boson Fusion and Decaying into a Pair of W and Z Bosons using 13 TeV pp Collision Data Recorded with the ATLAS Detector", was published by the CMS Collaboration in 2016. The study used data collected from proton-proton collisions at $\sqrt{s} =$ 8 TeV, corresponding to an integrated luminosity of 19.7 fb$^{-1}$.
Theory
According to the Standard Model of particle physics, there are three generations of quarks: up/down, charm/strange, and top/bottom. Each generation consists of two quarks with opposite charge but otherwise identical properties. The top quark is unique as it is significantly heavier than all other known particles except for its antiparticle partner, making it an important area for research.
One interesting possibility beyond the Standard Model is that there may be additional types or "flavors" of quarks that have not yet been discovered. This could include a fourth generation or exotic particles like charged Higgs bosons which are predicted by some theories but have not yet been observed.
Charged Higgs bosons are hypothetical particles that carry electric charge unlike their neutral counterparts in the Standard Model. They play a crucial role in many extensions to this model including supersymmetry and models with extra dimensions.
Methodology
To search for the presence of a charged Higgs boson, the CMS Collaboration looked for events with at least four jets, a charged lepton (muon or electron), and missing transverse momentum as signatures of the decay process. The data was analyzed using a kinematic fit to distinguish between jets likely originating from direct top quark decays and those potentially associated with the decay of the charged Higgs boson.
The researchers also used Monte Carlo simulations to model background processes that could mimic their signal. These included standard model processes such as t$\overline{\mathrm{t}}$ production, W/Z+jets production, and single top quark production. The simulated events were then compared to the observed data to determine if there were any discrepancies that could indicate the presence of a charged Higgs boson.
Results
After analyzing the data, no evidence for the presence of a charged Higgs boson was found. Upper limits at 95% confidence level were set on the branching fraction $\mathcal{B}$(t $\to$ H$^+$b) in different scenarios involving varying numbers of b-tagged jets and different charged Higgs boson masses (MH+). These limits ranged from 0.8-0.5%, assuming $\mathcal{B}$(H$^+\to$c$\overline{\mathrm{b}}) =$1.0 and conservation of total branching fractions for top quark decays.
Figure 2 in the paper illustrates post-fit dijet mass distributions from standard model backgrounds compared with expected signal yields for different scenarios involving varying numbers of b-tagged jets and different charged Higgs boson masses (MH+). This figure shows good agreement between data and simulation, further supporting their conclusion that no significant excess is present in their data.
Conclusion
In conclusion, the CMS Collaboration conducted a search for charged Higgs bosons decaying to charm and bottom quarks using 19.7 fb$^{-1}$ of proton-proton collision data at $\sqrt{s} =$ 8 TeV. Despite their efforts, no evidence for the presence of a charged Higgs boson was found in the data.
This study provides valuable insights into the search for exotic particles in high-energy collisions at CERN's Large Hadron Collider facility. It also sets important limits on the branching fraction of top quark decays to a charged Higgs boson and b-quark, which can be used to constrain various theoretical models beyond the Standard Model.
Further studies with larger datasets and higher energies are needed to continue this search and potentially shed light on new physics beyond our current understanding. The CMS Collaboration continues to push the boundaries of particle physics research, bringing us closer to unraveling some of the mysteries of our universe.
A search for charged Higgs boson decaying to a charm and a bottom quark (H$^+\to$ c$\overline{\mathrm{b}}$) is performed using 19.7 fb$^{-1}$ of pp collision data at $\sqrt{s} =$ 8 TeV. The production mechanism investigated in this search is $\mathrm{t\overline{t}}$ pair production in which one top quark decays to a charged Higgs boson and a bottom quark and the other decays to a charged lepton, a neutrino, and a bottom quark. Charged Higgs boson decays to $\mathrm{c\overline{b}}$ are searched for, resulting in a final state containing at least four jets, a charged lepton (muon or electron), and missing transverse momentum. A kinematic fit is performed to identify the pair of jets least likely to be the bottom quarks originating from direct top quark decays and the invariant mass of this pair is used as the final observable in the search. No evidence for the presence of a charged Higgs boson is observed and upper limits at 95% confidence level of 0.8-0.5% are set on the branching fraction $\mathcal{B}$(t $\to$ H$^+$b), assuming $\mathcal{B}$(H$^+\to$ $\mathrm{c\overline{b}}) =$ 1.0 and $\mathcal{B}$(t $\to$ H$^+$b) $+$ $\mathcal{B}$(t $\to$ Wb) = 1.0, for the charged Higgs boson mass range 90-150 GeV.
