[Paper Review] High energy single top photoproduction at the LHC

S. Ovyn, J. De Favereau De Jeneret|arXiv (Cornell University)|Jun 30, 2008

Particle physics theoretical and experimental studies1 references3 citations

TL;DR

This paper proposes using high-energy photon-proton collisions at the LHC to study top quark properties via single top photoproduction, employing rapidity gap and exclusivity tagging to suppress background. With 1 fb⁻¹ at very low luminosity and 30 fb⁻¹ at low luminosity, it achieves improved limits on anomalous FCNC couplings: $k_{tu\gamma} < 0.044$ and $k_{tc\gamma} < 0.077$ in the very low phase, and $k_{tu\gamma} < 0.029$, $k_{tc\gamma} < 0.050$ in the low phase, surpassing HERA limits by a factor of 3–4.

ABSTRACT

High-energy photon-proton interactions at the LHC offer interesting possibilities for the study of top properties. Using a fast simulation of a LHC-like detector, first results on the measurement of the |V_{tb}| matrix element using Wt photoproduction are presented. Anomalous photoproduction of single top due to Flavour-Changing Neutral Currents permits to improve the current limits on the coupling parameters, k_{tuγ} and k_{tcγ} after only 1 fb^{-1}.

Motivation & Objective

• To measure the $|V_{tb}|$ CKM matrix element using $Wt$ photoproduction at the LHC.
• To probe anomalous Flavour-Changing Neutral Current (FCNC) couplings $k_{tu\gamma}$ and $k_{tc\gamma}$ via single top photoproduction.
• To assess the feasibility of extracting high-energy photon-induced processes from dominant $pp$ backgrounds using tagging techniques.
• To evaluate the sensitivity of the LHC to anomalous couplings using realistic detector simulations and luminosity scenarios.

Proposed method

• Simulated events for SM and anomalous single top photoproduction were generated using MadGraph/MadEvent and CalcHEP.
• Events were passed through Pythia 6.227 for parton showering and a fast detector simulation to model detector response and acceptance.
• Signal selection relied on rapidity gap (lrg) and exclusivity cuts to suppress $pp$ backgrounds, especially in low-luminosity conditions.
• In high-luminosity scenarios, very forward detectors (VFDs) at 220 m and 420 m were used to tag escaping protons and improve background rejection.
• A kinematic consistency cut was applied by comparing the longitudinal momentum of the top quark reconstructed from central events and from proton energy loss.
• Systematic uncertainties were estimated using Monte Carlo convolution, including jet energy scale, rapidity gap, exclusivity, luminosity, theoretical, and b-tagging errors.

Experimental results

Research questions

• RQ1 Can the $|V_{tb}|$ CKM matrix element be measured with 10–17% precision using $Wt$ photoproduction at the LHC?
• RQ2 What are the expected limits on anomalous FCNC couplings $k_{tu\gamma}$ and $k_{tc\gamma}$ after 1 fb⁻¹ of integrated luminosity?
• RQ3 How effective are rapidity gap and exclusivity tagging in suppressing $pp$ background in low-luminosity conditions?
• RQ4 Can very forward detectors (VFDs) provide sufficient background rejection in high-luminosity scenarios to enable signal extraction?
• RQ5 How do systematic uncertainties, particularly from rapidity gap and exclusivity, affect the final sensitivity to anomalous couplings?

Key findings

• At very low luminosity (1 fb⁻¹), the signal rate is 83.2 ± 9.1 (stat.) ± 7.4 (syst.) events with a background of 12.7 ± 3.6 (stat.) ± 1.6 (syst.) events.
• At low luminosity (30 fb⁻¹), the signal rate increases to 1554 ± 39 (stat.) ± 138 (syst.) events, with a background of 327 ± 18 (stat.) ± 30 (syst.) events.
• The expected limit on $k_{tu\gamma}$ is $< 0.044$ and on $k_{tc\gamma}$ is $< 0.077$ at 95% C.L. in the very low luminosity phase.
• In the low luminosity phase, the limits improve to $k_{tu\gamma} < 0.029$ and $k_{tc\gamma} < 0.050$ at 95% C.L., representing a 3–4× improvement over HERA limits.
• The rapidity gap and exclusivity cuts reduce $pp$ backgrounds by more than $10^{-3}$ in low-luminosity conditions.
• VFDs at 220 m and 420 m provide rejection factors of 11 and 5.6 at $10^{33}$ and $2\times10^{33}$ cm⁻²s⁻¹ luminosity, respectively, enabling signal extraction in high-pile-up environments.