Vector leptoquarks (vLQs) are popular candidates for searching for physics beyond the Standard Model (SM). In this paper, we present updated exclusion limits on various vLQ species, accounting for all the relevant production mechanisms at the LHC. In particular, we highlight the critical role of indirect production and its interference with the SM Drell-Yan process. This interference can be constructive or destructive, depending on the specific quantum numbers of the vLQ, and significantly impacts the sensitivity of current searches. Furthermore, we demonstrate that including <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mrow><mi>QCD</mi><mo stretchy="false">+</mo></mrow><mi>QED</mi></mrow></math> mixed pair production channels leads to a noticeable shift in model-independent mass limits. Additionally, we examine the validity of the full theory with vLQs and corresponding effective operators in the high mass regime. Overall, our analysis yields a substantial improvement in the exclusion limits on vLQs compared to the existing results in the literature.

India

Arijit Das

Tanumoy Mandal

Subhadip Mitra

Rachit Sharma

Fresh look at the LHC limits on vector leptoquarks

We use a hybrid deep neural network (HDNN) to identify a boosted dark photon jet as a signature of a heavy vectorlike fermionic portal matter (PM) connecting the visible and the dark sectors. In this work, the fermionic PM, which mixes only with the Standard Model (SM) third-generation up-type quark, predominantly decays into a top quark and a dark photon pair. The dark photon then promptly decays to a pair of standard model fermions via the gauge kinetic mixing. We have analyzed two different final states, namely, (i) exactly one tagged dark photon and exactly one tagged top quark jet, and (ii) at least two tagged dark photons and at least one tagged top quark jet at the 13 and 14 TeV LHC center of mass energies. Both these final states receive significant contributions from the pair and single production processes of the top partner. The rich event topology of the signal processes, i.e., the presence of a boosted dark photon and top quark jet pair, along with the fact that the invariant mass of the system corresponds to the mass of the top partner, help us to significantly suppress potential SM backgrounds. We have shown that one can set a <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mn>2</mn><mi>σ</mi></mrow></math> exclusion limit of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mo>∼</mo><mn>2.3</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></math> on the top partner mass with <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>sin</mi><msub><mi>θ</mi><mi>L</mi></msub><mo>=</mo><mn>0.1</mn></math> and assuming 100% branching ratio of the top partner in the final state with exactly one tagged dark photon and exactly one tagged top quark jet at the 14 TeV LHC center of mass energy assuming <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>300</mn><mtext> </mtext><mtext> </mtext><msup><mi>fb</mi><mrow><mo>−</mo><mn>1</mn></mrow></msup></math> of integrated luminosity.

Shivam Verma

Sanjoy Biswas

Machine learning tagged boosted dark photon: A signature of fermionic portal matter at the LHC

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>.

Jai Bardhan

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

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.

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

The leading order production of an SM singlet-like scalar has primarily been realized through the gluon fusion process by mixing with the <math><mi>S</mi><mi>U</mi><msub><mrow><mo>(</mo><mn>2</mn><mo>)</mo></mrow><mrow><mi>L</mi></mrow></msub></math> scalar doublet of the model. The dominant part of the physical state, i.e., the singlet component, does not have any role in its direct production. Focusing on such a state with a mass smaller than the SM-like Higgs scalar, we calculate the dominant next-to-leading (NLO) order corrections to its production cross-section. With these improved cross-sections, the present and future LHC limits may become somewhat more stringent.