We study the lepton flavor conserving and violating couplings of a doubly charged scalar at a 3 TeV muon collider. Using a model-independent Lagrangian, we analyze the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>e</mi><mi>e</mi></math>, <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>μ</mi><mi>μ</mi></math>, and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>τ</mi><mi>τ</mi></math> final states mediated by the doubly charged scalar to probe individual couplings to <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>μ</mi><mi>e</mi></math>, <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>μ</mi><mi>μ</mi></math>, and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>μ</mi><mi>τ</mi></math>. We find that for a doubly charged scalar of mass greater than 1 TeV and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="script">O</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></math> couplings, we achieve high signal significance in these channels. We delineate the collider’s sensitivity in the mass-coupling plane, highlighting the extensive reach of the muon collider in probing these couplings far beyond the current experimental limits. We also propose an angular distribution variable to discriminate between the exchange of a doubly charged scalar from that of a neutral scalar, which gives identical signals.

Japan

Nivedita Ghosh

India

Santosh Kumar Rai

Tousik Samui

Agnivo Sarkar

Investigating the leptonic couplings of doubly charged scalars at the muon collider

We investigate the possibility of having a vector boson dark matter in a <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>U</mi><mo stretchy="false">(</mo><mn>1</mn><msub><mo stretchy="false">)</mo><mi>X</mi></msub></math> extended two-Higgs doublet model (2HDM) setup. The gauge boson gains mass when a Standard Model (SM) singlet complex scalar, which is charged under the dark <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>U</mi><mo stretchy="false">(</mo><mn>1</mn><msub><mo stretchy="false">)</mo><mi>X</mi></msub></math> symmetry, acquires a vacuum expectation value. This scalar acts as the connection between the SM sector and DM via the Higgs portal. An additional exact charge conjugation symmetry inhibits the mixing of this gauge boson with the photon, thereby confirming the stability of DM. On the other hand, 2HDM with type-I <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>Z</mi><mn>2</mn></msub></math> restriction can offer a nonstandard Higgs in the lighter mass range. This freedom allows us to accommodate dark matter mass in the (40–60)-GeV regime where the direct detection constraints are strongest. We study the dark matter phenomenology of such a model while taking care of all possible theoretical and experimental constraints.

Nandini Das

United States

Juhi Dutta

Dilip Kumar Ghosh

Vector dark matter in a <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>U</mi><mo stretchy="false">(</mo><mn>1</mn><msub><mo stretchy="false">)</mo><mi>X</mi></msub></math> extended 2HDM

We consider a neutrinophilic <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>U</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></math> extension of the Standard Model (SM) under which only a second Higgs doublet and SM singlet scalars and fermions are charged. The new gauge boson <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math> couples to the SM minimally, generated by <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Z</mi><mo>−</mo><msup><mi>Z</mi><mo>′</mo></msup></math> mixing. As the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math> is very weakly coupled, it can mostly be produced through the decay of the scalars from the second Higgs doublet at the Large Hadron Collider (LHC). We discuss the scalar sector of the model in detail and consider decay modes such as <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mo stretchy="false">(</mo><msup><mi>H</mi><mo>±</mo></msup><mo stretchy="false">→</mo><msup><mi>W</mi><mo>±</mo></msup><msup><mi>Z</mi><mo>′</mo></msup><mo>,</mo><msub><mi>h</mi><mn>2</mn></msub><mo stretchy="false">→</mo><mi>V</mi><mi>V</mi><mo>,</mo><mo stretchy="false">(</mo><mi>V</mi><mo>=</mo><msup><mi>W</mi><mo>±</mo></msup><mo>,</mo><mi>Z</mi><mo>,</mo><msup><mi>Z</mi><mo>′</mo></msup><mo stretchy="false">)</mo><mo>,</mo><msub><mi>A</mi><mn>2</mn></msub><mo stretchy="false">→</mo><msub><mi>h</mi><mn>1</mn></msub><msup><mi>Z</mi><mo>′</mo></msup><mo stretchy="false">(</mo><mi>Z</mi><mo stretchy="false">)</mo><mo stretchy="false">)</mo></math> that lead to multilepton signals at the LHC from the pair production of the scalars. We analyze the signal with a representative value of the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math> mass to show the discovery potential of the two-Higgs doublet model scalars at the LHC.

Anjan Kumar Barik

LHC signals for a 2HDM with <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math> in a neutrinophilic <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>U</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></math> model

One of the standard ways to study scenarios beyond the Standard Model involves extending the Higgs sector. This work examines the three Higgs doublet model (3HDM) in a type-Z or democratic setup, where each Higgs doublet couples exclusively to a specific type of fermion. The particle spectrum of the 3HDM includes four charged Higgs bosons, two <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>C</mi><mi>P</mi></mrow></math>-odd scalars, and three <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>C</mi><mi>P</mi></math>-even scalars. This work investigates the allowed mass and coupling parameter space in the type-Z 3HDM after imposing all theoretical and experimental constraints. We extract the allowed parameter space under three distinct alignment-limit conditions or mass hierarchies leveraging machine learning techniques. Specifically, we analyze scenarios where the 125 GeV Higgs is the lightest, an intermediary, or the heaviest <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>C</mi><mi>P</mi></math>-even Higgs boson. Our findings indicate that while a single lighter <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>C</mi><mi>P</mi></math>-even Higgs boson below 125 GeV still remains a possibility, the presence of two lighter Higgses is ruled out.

Nipun Batra

Baradhwaj Coleppa

Akshat Khanna

Constraining the 3HDM parameter space using active learning

We show in this letter how a heavy <math altimg='si1.svg'><mo stretchy='false'>(</mo><mi mathvariant='script'>O</mi><mo stretchy='false'>(</mo><mi>T</mi><mi>e</mi><mi>V</mi><mo stretchy='false'>)</mo><mo stretchy='false'>)</mo></math> invisible <math altimg='si2.svg'><msup><mrow><mi>Z</mi></mrow><mrow><mo>&#8242;</mo></mrow></msup></math> gauge boson that will practically be out of reach of the Large Hadron Collider (LHC), can be discovered at the future muon collider. The new force carrier has a relatively stronger coupling with the beyond standard model (BSM) sector, while its interaction with the SM fields is much weaker. This weaker coupling is induced through mixing mechanisms, specifically via gauge kinetic mixing and the <math altimg='si3.svg'><mi>Z</mi><mo linebreak='goodbreak' linebreakstyle='after'>&#8722;</mo><msup><mrow><mi>Z</mi></mrow><mrow><mo>&#8242;</mo></mrow></msup></math> mixing. We consider a scenario where the new gauge boson decays mostly to charge-neutral long-lived particles and/or dark matter (DM). We show how producing and detecting this heavier invisible <math altimg='si2.svg'><msup><mrow><mi>Z</mi></mrow><mrow><mo>&#8242;</mo></mrow></msup></math>, that will be beyond the reach of even the very high luminosity LHC, becomes possible if it is produced in association with an energetic photon at the future muon collider. The on-shell production of the <math altimg='si2.svg'><msup><mrow><mi>Z</mi></mrow><mrow><mo>&#8242;</mo></mrow></msup></math> will lead to a peak in the photon energy distribution, following the so-called radiative return phenomena and can lead to the accurate determination of the <math altimg='si2.svg'><msup><mrow><mi>Z</mi></mrow><mrow><mo>&#8242;</mo></mrow></msup></math> mass and its interaction with SM particles.

Aviral Srivastava

Discovering an invisible <italic>Z</italic>&#8242; at the muon collider

We study a scotogenic model augmented with an additional <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>U</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></math> gauge and a discrete <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>Z</mi><mn>2</mn></msub></math> symmetry. The lightest <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>Z</mi><mn>2</mn></msub></math>-odd particle in our model becomes the dark matter (DM) candidate while tiny neutrino masses are realized at one loop. We explore the parameter space of the model for which the DM relic density is satisfied, and the correct low-energy neutrino observables are reproduced. The extended gauge symmetry includes a beyond Standard Model (SM) particle spectrum consisting of vectorlike fermions and scalars. We also highlight possible collider signatures of these particles at the LHC.

Najimuddin Khan

Scotogenic model with U(1) symmetry and a scalar dark matter

The proposal for a high-energy muon collider offers many opportunities in the search for physics beyond the Standard Model (BSM). The collider by construction is likely to be more sensitive to the muon-philic models, primarily motivated by the BSM explanation of muon <math><mo>(</mo><mi>g</mi><mo>−</mo><mn>2</mn><mo>)</mo></math> excess and quark flavor anomalies. In this work, we explore the potential of the proposed muon collider in the context of such models and focus on one such model that extends the Standard Model (SM) with a leptoquark, a vector-like lepton, and a real scalar. In this model, we propose searches for TeV scale leptoquarks in <math><mn>2</mn><mi>μ</mi><mo>+</mo><mn>2</mn><mi>b</mi><mo>+</mo><msub><mrow><mi>E̸</mi></mrow><mrow><mi>T</mi></mrow></msub></math> channel. Notably, the leptoquark can be produced singly at the muon collider with a large cross-section. We have shown that a significant signal in this channel can be detected at a 3 TeV muon collider even with an integrated luminosity as low as ∼10 fb$^{−1}$.

Search for a leptoquark and vector-like lepton in a muon collider

Despite the immense success of the Standard Model (SM), the hunt for physics beyond the Standard Model (BSM) has continued. Extension of the SM gauge group or the particle content of the SM remains a viable solution to many observed anomalies, such as the anomalous magnetic moment of muon, flavor violation, dark matter, etc. In this work, we consider a BSM model which includes a leptoquark, a vectorlike lepton, and a real scalar singlet. The model accounts for a dark matter candidate through a <math><msub><mi>Z</mi><mn>2</mn></msub></math> symmetry in addition to predicting the muon (<math><mrow><mi>g</mi><mo>−</mo><mn>2</mn></mrow></math>) in agreement with the experimental measurement. We also show that the experimental measurements of the lepton flavor violation are satisfied in a wide range of parameter space. The viable parameter space of the model is used to study the collider signatures arising from the pair production of the leptoquark in the <math><mrow><mn>2</mn><mi>μ</mi><mo>+</mo><mn>2</mn><mi>b</mi><mo>+</mo><msub><mrow><mrow><mi>E</mi></mrow></mrow><mrow><mi>T</mi></mrow></msub></mrow></math> channel at the 14 TeV LHC run.

Collider signatures of a scalar leptoquark and vectorlike lepton in light of muon anomaly

We investigate the possibility to identify the intrinsic spin of exotic charged particles at a future <math><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup></math> collider in the <math><msup><mi>l</mi><mo>±</mo></msup><mo>+</mo><mn>2</mn><mi>j</mi><mo>+</mo><msub><mi>E</mi><mi>T</mi></msub></math> final state. We choose the inert doublet model (IDM) and minimal supersymmetric standard model (MSSM) as examples for the new physics models with scalar and fermionic exotic charged particles, respectively. The signal arises when these exotic charged particles are pair produced and then decay to a <math><msup><mi>W</mi><mo>±</mo></msup></math> boson and the lightest neutral stable particle in the new physics model. We choose four benchmarks for the mass parameters which give significant deviation from the dominant standard model (SM) <math><msup><mi>W</mi><mo>+</mo></msup><msup><mi>W</mi><mo>−</mo></msup></math> background. We find that an asymmetry in the cosine of the scattering angle (<math><mi>cos</mi><mi>θ</mi></math>) of one of the <math><mi>W</mi></math> bosons reconstructed from <math><mi>j</mi><mi>j</mi></math> pair as well as the charged lepton have the potential to identify the MSSM and IDM signal over SM with longitudinally polarized initial beams. A more robust distinction is seen in the shape of the azimuthal angle distribution of the <math><mi>W</mi></math> boson and charged lepton, which can identify and distinguish the IDM signal from MSSM further if the initial beams are transversely polarized.

Rafiqul Rahaman

Distinguishing nonstandard scalar and fermionic charged particles at a future <math><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup></math> collider

We show how a new hidden gauge symmetry responsible for neutrino mass as well as dark matter in the Universe can be discovered through scalar mediators responsible for breaking the new symmetry. The new force mediator (<math><mrow><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></mrow></math>) may be lighter than the Standard Model gauge bosons but cannot be observed in traditional searches for new gauge bosons. We highlight a novel way of discovering such a symmetry at the Large Hadron Collider by incorporating an existing ATLAS analysis on four-lepton final states which include the Higgs resonance. In addition, we show that the hidden sector also introduces flavor violation in the lepton sector, which can become a significant channel of discovery for the new force.