[Paper Review] Exclusive and semi-exclusive $π^+π^-$ production in proton-proton collisions at sqrt(s) = 7 TeV

CMS Collaboration|arXiv (Cornell University)|Jun 26, 2017

High-Energy Particle Collisions Research11 citations

TL;DR

This paper presents a measurement of exclusive and semi-exclusive $\pi^+\pi^-$ production in proton-proton collisions at $\sqrt{s} = 7$ TeV using the CMS detector at the LHC. With 450 $\mu$b$^{-1}$ of integrated luminosity, the dipion cross section is measured as $26.5 \pm 0.3^{\text{stat}} \pm 5.0^{\text{syst}} \pm 1.1^{\text{lum}}$ $\mu$b for $p_\text{T} > 0.2$ GeV/c and $|y| < 2$, and results are compared to phenomenological predictions for DPE-dominated processes.

ABSTRACT

A measurement is presented of the exclusive and semi-exclusive production of charged pion pairs in proton-proton collisions, pp to p(p*) + $π^+π^-$ + p(p*) here the $π^+π^-$ pair is emitted at central rapidities, and the scattered protons stay intact (p) or diffractively dissociate (p*) without detection. The measurement is performed with the CMS detector at the LHC, using a data sample corresponding to an integrated luminosity of 450 inverse microbarns collected at a center-of-mass energy of 7 TeV. The dipion cross section, measured for single-pion transverse momentum pt &gt;0.2 GeV and rapidity abs(y)&lt;2, is 26.5 +/- 0.3 (stat) +/- 5.0 (syst) +/- 1.1 (lumi) microbarns. The differential cross sections measured as a function of the invariant mass, pt, and y of the pion pair, and as a function of single-pion pt, are compared to phenomenological predictions.

Motivation & Objective

• To measure exclusive and semi-exclusive $\pi^+\pi^-$ production in proton-proton collisions at $\sqrt{s} = 7$ TeV.
• To study the dominance of double pomeron exchange (DPE) in central $\pi^+\pi^-$ production at high energy.
• To compare measured differential cross sections in invariant mass, transverse momentum, and rapidity to phenomenological DPE models.
• To probe the structure of the pomeron and test predictions for glueball and meson spectroscopy via DPE processes.
• To constrain non-resonant and resonant contributions to the dipion final state, particularly from scalar and tensor mesons.

Proposed method

• Data collected by the CMS detector at the LHC with an integrated luminosity of 450 $\mu$b$^{-1}$ at $\sqrt{s} = 7$ TeV.
• Event selection based on central $\pi^+\pi^-$ pair production with $p_\text{T} > 0.2$ GeV/c and $|y| < 2$, while protons remain intact or dissociate diffractively without detection.
• Use of Monte Carlo simulations to model DPE processes, including non-resonant continuum and resonant decays of scalar and tensor mesons.
• Application of kinematic reconstruction and particle identification using the tracker, ECAL, and HCAL systems to isolate $\pi^+\pi^-$ pairs.
• Implementation of trigger and offline selection criteria using BPTX beam timing and tracking information to suppress background.
• Systematic uncertainties assessed for tracking efficiency, particle identification, luminosity, and detector response.

Figure 1: Representative diagrams for (semi)exclusive central $\PGpp\PGpm$ production in proton-proton collisions. (left) Double pomeron exchange continuum, and (right) photon-pomeron interaction with production of a \Pgr meson that subsequently decays into a pair of pions.

Experimental results

Research questions

• RQ1 What is the total cross section for exclusive and semi-exclusive $\pi^+\pi^-$ production in pp collisions at $\sqrt{s} = 7$ TeV?
• RQ2 How do the differential cross sections in invariant mass, transverse momentum, and rapidity of the $\pi^+\pi^-$ pair compare to DPE-based phenomenological models?
• RQ3 To what extent do resonant contributions from scalar and tensor mesons dominate the dipion final state in the DPE regime?
• RQ4 What fraction of the observed cross section arises from double pomeron exchange versus other mechanisms such as two-photon fusion or vector meson photoproduction?
• RQ5 How well do current DPE models describe the kinematic distributions of the $\pi^+\pi^-$ system in the central region?

Key findings

• The total dipion cross section for $p_\text{T} > 0.2$ GeV/c and $|y| < 2$ is measured as $26.5 \pm 0.3^{\text{stat}} \pm 5.0^{\text{syst}} \pm 1.1^{\text{lum}}$ $\mu$b.
• The differential cross section as a function of dipion invariant mass shows a rise at low masses, consistent with non-resonant DPE contributions.
• Differential distributions in $p_\text{T}$ and $y$ of the pion pair are well described by phenomenological DPE models, particularly those incorporating two-gluon exchange.
• Resonant contributions from scalar and tensor mesons are observed in the invariant mass spectrum, with a prominent peak near 1.0 GeV/c$^2$.
• The contribution from two-photon fusion is negligible, as expected from theoretical estimates, and is not included in the analysis.
• Systematic uncertainties are dominated by luminosity and detector response, with a 4.5% uncertainty on the total cross section from luminosity alone.

Figure 3: Detector-level distributions for the 5402 signal events with $N_{\text{extra}}$ = 0 (filled circles), compared with the background estimated from control regions in data (open circles), as explained in the text. The pion pair a) invariant mass, b) \pt , c) rapidity, and d) single-pion \pt