Following up on our earlier study [J. Bardhan et al., <a href="http://dx.doi.org/10.1103/PhysRevD.107.115001">Phys. Rev. D 107, 115001 (2023)</a>], we investigate the LHC prospects of pair-produced vectorlike <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>B</mi></mrow></math> quarks decaying exotically to a new gauge-singlet (pseudo)scalar field <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="normal">Φ</mi></math> and a <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>b</mi></math> quark. After the electroweak symmetry breaking, the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="normal">Φ</mi></math> decays predominantly to <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>g</mi><mi>g</mi><mo>/</mo><mi>b</mi><mi>b</mi></math> final states, leading to a fully hadronic <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>2</mn><mi>b</mi><mo>+</mo><mn>4</mn><mi>j</mi></math> or <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>6</mn><mi>b</mi></math> signature. Because of the large Standard Model background and the lack of leptonic handles, it is a difficult channel to probe. To overcome the challenge, we employ a hybrid deep learning model containing a graph neural network followed by a deep neural network. We estimate that such a state-of-the-art deep learning analysis pipeline can lead to a performance comparable to that in the semileptonic mode, taking the discovery (exclusion) reach up to about <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi>M</mi></mrow><mrow><mi>B</mi></mrow></msub><mo>=</mo><mn>1.8</mn><mo stretchy="false">(</mo><mn>2.4</mn><mo stretchy="false">)</mo><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math> at HL-LHC when <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>B</mi></math> decays fully exotically, i.e., <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>BR</mi><mo stretchy="false">(</mo><mi>B</mi><mo stretchy="false">→</mo><mi>b</mi><mi mathvariant="normal">Φ</mi><mo stretchy="false">)</mo><mo>=</mo><mn>100</mn><mo>%</mo></mrow></math>.

India

Jai Bardhan

Tanumoy Mandal

Subhadip Mitra

Cyrin Neeraj

Mihir Rawat

Tagging fully hadronic exotic decays of the vectorlike <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>B</mi></mrow></math> quark using a graph neural network

The collider phenomenology of leptoquarks (LQs) and right-handed neutrinos (RHNs) has been studied extensively in the literature. Due to their gauge singlet nature, the production of RHNs at the LHC is typically suppressed by the tiny light-heavy neutrino mixing angles. In this study, we explore a promising scenario where the presence of a LQ mediator significantly enhances the RHN production: the LQ essentially couples only to first-generation fermions (quarks and leptons, including the RHN), and the coupling with the first-generation RHN dominates. We consider all possible scalar and vector LQs that could lead to such a scenario. The prospects are better for the first-generation scenario than the other generations because of the enhanced parton distribution functions (PDFs) of first-generation quarks. The enhanced PDFs boost the production cross sections of LQs, particularly their single and indirect productions. Incorporating all production modes of LQs that result in a pair of RHNs, we estimate the discovery prospects by analyzing the monoelectron and dielectron channels arising from the decay of the RHN pair. We find that the indirect production of LQs is crucial in determining the discovery reach at the HL-LHC for the first-generation scenario.

Gokul Duraikandan

Rishabh Khanna

Rachit Sharma

Right-handed neutrino production through first-generation leptoquarks

Leptoquarks (LQs) can contribute to the magnetic and electric dipole moments of charged leptons, which the current experiments have measured with good accuracy. We revisit the parameter spaces of TeV-scale vector LQs that contribute to these observables and study how these models fare against the LHC bounds. We show that significant portions of the parameter space are excluded when the current LHC data is utilized effectively. We find that only <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>U</mi><mn>1</mn></msub></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>V</mi><mn>2</mn></msub></math> can explain the observed positive shift in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mo stretchy="false">(</mo><msubsup><mi>a</mi><mi>μ</mi><mi>exp</mi></msubsup><mo>−</mo><msubsup><mi>a</mi><mi>μ</mi><mrow><mi>SM</mi></mrow></msubsup><mo stretchy="false">)</mo></math> through a lepton chirality-flipping contribution with <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> LQ-quark-lepton coupling. We also see how these two LQs can fit the electron dipole moment and atomic parity violation measurements. We find that the current electric dipole moment measurements of the muon cannot restrain the LQ-quark-lepton couplings within perturbative regions.

Arvind Bhaskar

Diganta Das

Soumyadip Kundu

United States

Anirudhan A. Madathil

Vector leptoquark contributions to lepton dipole moments

The scalar-leptoquark (sLQ) parameter space is well explored experimentally. The direct pair production searches at the LHC have excluded light sLQs almost model agnostically, and the high-<math><msub><mi>p</mi><mi>T</mi></msub></math> dilepton tail data have put strong bounds on the leptoquark-quark-lepton Yukawa couplings for a wide range of sLQ masses. However, these do not show the complete picture. Previously, Mandal et al. [Single productions of colored particles at the LHC: An example with scalar leptoquarks, J. High Energy Phys. 07 (2015) 028] showed how the dilepton-dijet data from the pair production searches could give strong limits on these couplings. This was possible by including the single-production contribution to the dilepton-dijet signal. In this paper, we take a fresh look at the LHC limits on all sLQs by following the same principle and combine all significant contributions—from pair and single productions, <math><mi>t</mi></math>-channel sLQ exchange and its interference with the Standard Model background—to the <math><mi>μ</mi><mi>μ</mi><mi>j</mi><mi>j</mi></math> final state and recast the limits. We notice that the sLQ exchange and its interference with the background processes play significant roles in the limits. The <math><mi>μ</mi><mi>μ</mi><mi>j</mi><mi>j</mi></math>-recast limits are comparable to or, in some cases, significantly better than the currently known limits (from high-<math><msub><mi>p</mi><mi>T</mi></msub></math> dilepton data and direct searches), i.e., the LHC data rules out more parameter space than what is considered in the current literature. For the first time, we also show how including the QED processes can noticeably improve the sLQ mass exclusion limits from the QCD-only limits.

Arijit Das

Fresh look at the LHC limits on scalar leptoquarks

A leptophobic <math><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></math> that does not couple with the Standard Model leptons can evade the stringent bounds from the dilepton-resonance searches. In our earlier paper [T. Arun et al., Search for the <math><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></math> boson decaying to a right-handed neutrino pair in leptophobic <math><mi>U</mi><mo>(</mo><mn>1</mn><mo>)</mo></math> models, Phys. Rev. D, 106 (2022) 095035; arXiv:2204.02949], we presented two gauge anomaly-free <math><mi>U</mi><mo>(</mo><mn>1</mn><mo>)</mo></math> models—one based on the Green-Schwarz (GS) anomaly cancellation mechanism, and the other on a grand unified theory (GUT) framework with gauge kinetic mixing—where a heavy leptophobic <math><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></math> is present along with right-handed neutrinos (<math><msub><mrow><mi>N</mi></mrow><mrow><mi>R</mi></mrow></msub></math>). We pointed out the interesting possibility of a correlated search for <math><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></math> and <math><msub><mrow><mi>N</mi></mrow><mrow><mi>R</mi></mrow></msub></math> at the LHC through the <math><mi>p</mi><mi>p</mi><mo>→</mo><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup><mo>→</mo><msub><mrow><mi>N</mi></mrow><mrow><mi>R</mi></mrow></msub><msub><mrow><mi>N</mi></mrow><mrow><mi>R</mi></mrow></msub></math> channel. This channel can probe a part of the <math><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></math> parameter space beyond the reach of the standard dijet resonance searches. In this follow-up paper, we analyse the challenging monolepton final state arising from the decays of the <math><msub><mrow><mi>N</mi></mrow><mrow><mi>R</mi></mrow></msub></math> pair with Deep Learning. We present the high-luminosity LHC discovery reaches for six different GUT embeddings and a benchmark point in the GS setup. We also update our previous estimates in the dilepton channel with Deep Learning. We identify parameter regions that can be probed with the proposed channel but will remain inaccessible to dijet searches at the HL-LHC.

Aniket Masaye

Naveen Reule

Kalp Shah

Pinning down the leptophobic ′ in leptonic final states with Deep Learning

No direct experimental constraints exist on Leptoquark (LQ) couplings with quarks and right-handed neutrinos (RHNs). If a LQ dominantly couples to RHNs, it can leave unique signatures at the LHC. The RHNs can be produced copiously from LQ decays as long as they are lighter than the LQs. LQ-induced RHN production has never been searched for in experiments. This channel can act as a simultaneous probe for RHNs and LQs that dominantly couple to RHNs. In this paper, we consider all possible charge-2/3 and 1/3 scalar and vector LQs that dominantly couple to second-generation quarks and RHN. We study the pair and single productions of TeV-scale LQs and their subsequent decay to sub-TeV RHNs, realised in the inverse seesaw framework. We also consider RHN pair production through a t-channel LQ exchange. The single LQ production and t-channel contributions can be significant for large LQ-RHN-quark couplings. We systematically combine events from these processes leading to a pair of RHNs plus jets to study the prospects of LQ-assisted RHN pair production. We analyse the monolepton and opposite-sign dilepton final states and estimate the discovery reach at the high-luminosity LHC.

Yash Chaurasia

Kuldeep Deka

Ananya Mukherjee

Right-handed neutrino pair production via second-generation leptoquarks

The presence of a new decay mode relaxes the current mass exclusion limits on vectorlike quarks considerably. We consider the case of a weak-singlet vectorlike <math><mi>B</mi></math> quark that can decay to a singlet scalar or pseudoscalar <math><mi>Φ</mi></math>. In an earlier paper [A. Bhardwaj et al., Roadmap to explore vectorlike quarks decaying to a new scalar or pseudoscalar, Phys. Rev. D, 106, 095014 (2022)], we mapped the possibilities to explore such setups at the LHC. We showed that it is possible for a <math><mi>B</mi></math> quark to decay into <math><mi>Φ</mi></math> and the <math><mi>Φ</mi></math> to dominantly decay to a pair of gluons or <math><mi>b</mi></math> quark(s) without fine-tuning the parameters. In this paper, we present a collider search strategy to look for the pair production of singlet <math><mi>B</mi></math> quarks. If both <math><mi>B</mi></math>’s decay into <math><mi>b</mi><mi>Φ</mi></math> pairs, the final state is fully hadronic: <math><mi>B</mi><mi>B</mi><mo>→</mo><mo>(</mo><mi>b</mi><mi>Φ</mi><mo>)</mo><mo>(</mo><mi>b</mi><mi>Φ</mi><mo>)</mo><mo>→</mo><mo>(</mo><mi>b</mi><mi>g</mi><mi>g</mi><mo>)</mo><mo>(</mo><mi>b</mi><mi>g</mi><mi>g</mi><mo>)</mo><mo>/</mo><mo>(</mo><mi>b</mi><mi>b</mi><mi>b</mi><mo>)</mo><mo>(</mo><mi>b</mi><mi>b</mi><mi>b</mi><mo>)</mo></math>, which is very challenging to probe. Therefore, we consider a simpler mixed decay specific to the singlet <math><mi>B</mi></math> case, <math><mi>B</mi><mi>B</mi><mo>→</mo><mo>(</mo><mi>b</mi><mi>Φ</mi><mo>)</mo><mo>(</mo><mi>t</mi><mi>W</mi><mo>)</mo></math> with a lepton in the final state, to achieve the best sensitivity at the high-luminosity LHC. We use a deep neural network with a weighted cross-entropy loss to separate the signal from the huge Standard Model background. Our analysis shows that large areas of the <math><msub><mi>M</mi><mi>B</mi></msub><mo>−</mo><msub><mi>M</mi><mi>Φ</mi></msub></math> parameter space are discoverable through this signature. We show how the discovery and exclusion regions scale with the branching ratio in the new decay mode. We also estimate the reach in the inclusive monolepton channel with the same network model.

Machine learning-enhanced search for a vectorlike singlet <math><mi>B</mi></math> quark decaying to a singlet scalar or pseudoscalar

We explore an anomaly-free <math><mtext>U</mtext><mo>(</mo><mn>1</mn><mo>)</mo></math> gauge extended beyond the Standard model (BSM) framework, to account for the baryon asymmetry of the Universe, along with arranging for tiny neutrino mass. Neutrino masses are generated via higher-dimensional operators (HDOs) involving three right-handed neutrinos (RHNs) with gauge charges (4, 4 and −5 respectively) and two BSM scalars. This is an attractive framework as it can accommodate a keV scale dark matter, with the lightest RHN being the candidate. The remaining two RHNs are quasi-degenerate at the TeV-scale, actively participating in the process of resonant leptogenesis through their decay governed by the same set of HDOs. The RHNs being at the TeV scale, make this framework relevant for studying flavored resonant leptogenesis. This TeV-scale resonant leptogenesis, after satisfying the neutrino oscillation data, leads to interesting predictions on the Yukawa sector of the model. The thermal evolution of the baryon asymmetry has followed the experimental results rather accurately in that corner of parameter space. As a matter of fact, this TeV-scale framework which in principle relies on the low scale resonant leptogenesis typically leads to predictions that potentially can be tested at the colliders. In particular, we consider the same-sign dilepton signature that arises from the RHN pair production through the decay of heavy gauge boson of the extra <math><mtext>U</mtext><mo>(</mo><mn>1</mn><mo>)</mo></math>.

Soumya Sadhukhan

Leptogenesis in an anomaly-free U(1) extension with higher-dimensional operators

In addition to the long-standing anomalies seen in the muon <math><mi>g</mi><mo>−</mo><mn>2</mn></math>, <math><msub><mi>R</mi><msup><mi>K</mi><mrow><mo>(</mo><mo>*</mo><mo>)</mo></mrow></msup></msub></math>, and <math><msub><mi>R</mi><msup><mi>D</mi><mrow><mo>(</mo><mo>*</mo><mo>)</mo></mrow></msup></msub></math> observables by various independent experiments, the CDF Collaboration has found another significant one in the <math><mi>W</mi></math>-boson mass. These anomalies might be intertwined at a fundamental level and have a single new-physics explanation. In this paper, we present a simultaneous solution to these anomalies with two scalar leptoquarks of roughly equal mass—one a weak-singlet <math><msub><mi>S</mi><mn>1</mn></msub></math> and the other a weak-triplet <math><msub><mi>S</mi><mn>3</mn></msub></math>—which mix through a Higgs portal. The solution has only a few new couplings and TeV-scale leptoquarks, making the solution testable at the LHC. We put a lower and upper bound on the leptoquark masses in this setup from the current LHC data and the assumption that all new couplings are within the perturbative limit.

Combined explanation of <math><mi>W</mi></math>-mass, muon <math><mi>g</mi><mo>−</mo><mn>2</mn></math>, <math><msub><mi>R</mi><msup><mi>K</mi><mrow><mo>(</mo><mo>*</mo><mo>)</mo></mrow></msup></msub></math> and <math><msub><mi>R</mi><msup><mi>D</mi><mrow><mo>(</mo><mo>*</mo><mo>)</mo></mrow></msup></msub></math> anomalies in a singlet-triplet scalar leptoquark model

The <math><mi>U</mi><mo>(</mo><mn>1</mn><mo>)</mo></math> extensions of the Standard Model contain a heavy neutral gauge boson <math><msup><mi>Z</mi><mo>′</mo></msup></math>. If leptophobic, the boson can evade the stringent bounds from the dilepton resonance searches. We consider two theoretically well-motivated examples of leptophobic <math><mi>U</mi><mo>(</mo><mn>1</mn><mo>)</mo></math> extensions in which the <math><msup><mi>Z</mi><mo>′</mo></msup></math> decays to right-handed neutrinos (RHNs) with substantial branchings. The coexistence of a leptophobic <math><msup><mi>Z</mi><mo>′</mo></msup></math> and the RHNs opens up a new possibility of searching for these particles simultaneously through the production of a <math><msup><mi>Z</mi><mo>′</mo></msup></math> at the LHC and its decay to a RHN pair. For this decay to occur, the RHNs need to be lighter than the <math><msup><mi>Z</mi><mo>′</mo></msup></math>. Hence, we study this process in an inverse seesaw setup where the RHNs can be in the TeV range. However, in this case, they have a pseudo-Dirac nature, i.e., a RHN pair would produce only opposite-sign lepton pairs, as opposed to the Majorana-type neutrinos, which can produce both same- and opposite-sign lepton pairs. Hence, the final state we study has a same-flavor opposite-sign lepton pair plus hadronically decaying boosted <math><mi>W</mi></math> bosons. Our analysis shows that the high luminosity LHC can discover a TeV-scale leptophobic <math><msup><mi>Z</mi><mo>′</mo></msup></math> decaying via a RHN pair in a wide range of available parameters. Interestingly, large parameter regions beyond the reach of future dijet-resonance searches can be probed exclusively through our channel.