[Paper Review] Forward physics with tagged protons at the LHC: QCD and anomalous couplings

C. Royon|arXiv (Cornell University)|Oct 17, 2013

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper proposes using tagged protons in LHC experiments to probe the Pomeron's gluon and quark structure via dijet and gamma+jet production in double Pomeron exchange processes, achieving sensitivity to QCD evolution and Pomeron universality. It also explores searches for anomalous W/Z/γ couplings and Higgs boson production through central exclusive processes, with key results showing high sensitivity to gluon density at high momentum fractions using dijet mass fraction observables and potential for 10 ps timing resolution with advanced detectors.

ABSTRACT

We present some physics topics that can be studied at the LHC using proton tagging. We distinguish the QCD (Pomeron structure, BFKL analysis...) from the exploratory physics topics (HIggs boson, anomalous couplings between photons and $W/Z$ bosons

Motivation & Objective

• To test the universality of the Pomeron structure between ep and pp collisions by measuring its gluon and quark content at the LHC.
• To constrain QCD evolution of the Pomeron using dijet and gamma+jet production in double Pomeron exchange events.
• To explore rare processes such as Higgs boson production and anomalous couplings between photons and W/Z bosons via central exclusive production.
• To develop and validate proton tagging detectors with high precision timing and spatial resolution for background suppression and exclusive event identification.
• To enable high-sensitivity measurements of Pomeron gluon density at high β using dijet mass fraction distributions.

Proposed method

• Utilizes the Forward Physics Monte Carlo (FPMC) generator to simulate diffractive processes including single diffraction, double Pomeron exchange, and two-photon exchange.
• Employs tagged protons detected at 210 m (ATLAS) and 220 m (CMS) to identify exclusive events and measure diffractive masses.
• Analyzes dijet cross sections as a function of jet transverse momentum (pT) and dijet mass fraction to probe gluon density in the Pomeron.
• Uses the dijet mass fraction, defined as √(β₁β₂), to enhance sensitivity to gluon distributions at high β values.
• Deploys a fast timing system based on quartz Cerenkov detectors coupled to microchannel plate photomultipliers (MCP-PMT) for 10 ps resolution.
• Develops the SAMPIC readout chip using waveform sampling to achieve sub-10 ps timing resolution with 1 GHz bandwidth and low cost per channel.

Experimental results

Research questions

• RQ1 Can the gluon density in the Pomeron measured at HERA be confirmed at the LHC using dijet production in double Pomeron exchange?
• RQ2 How sensitive is the dijet mass fraction distribution to variations in the Pomeron's gluon density at high β values?
• RQ3 To what extent can proton tagging at 210–220 m improve the measurement of exclusive Higgs boson production and anomalous couplings?
• RQ4 What is the achievable timing resolution for proton tagging using quartz Cerenkov-based detectors and advanced readout electronics?
• RQ5 Can the SAMPIC chip achieve sub-10 ps timing resolution with high rate capability and low cost per channel?

Key findings

• The dijet cross section at the LHC is highly sensitive to the gluon density in the Pomeron, with measurable differences when the gluon density is varied by a factor of (1−β)^ν across ν = −1 to 1.
• The dijet mass fraction distribution shows significantly spaced curves for different ν values at high mass fractions (>0.6–0.7), indicating strong sensitivity to gluon density at high β.
• Exclusive dijet events contribute significantly to the high dijet mass fraction region, enhancing the observability of Pomeron gluon structure.
• The timing system using QUARTIC bars achieves a time resolution of ~34 ps per bar, with a projected resolution of ~10 ps using eight bars in parallel.
• The SAMPIC chip is designed to achieve sub-10 ps timing resolution, 1 GHz input bandwidth, and a cost of ~$10 per channel, enabling high-rate applications.
• The combination of proton tagging and fast timing allows for a factor of 40 rejection of pile-up background by constraining the event vertex.