[Paper Review] Baryon Production at LHC Experiments: Average pt of Hyperons vs. Energy

O. I. Piskounova|arXiv (Cornell University)|Jun 23, 2017

Particle physics theoretical and experimental studies21 references3 citations

TL;DR

This paper analyzes transverse momentum ($p_t$) spectra of hyperons in proton-proton and proton-antiproton collisions at LHC and previous colliders using the Quark-Gluon String Model (QGSM). It finds a steady increase in average $p_t$ of $\Lambda^0$ hyperons with center-of-mass energy up to $\sqrt{s} = 7$ TeV, with no significant change in baryon production dynamics, implying the cosmic ray 'knee' at $\sim 4 \times 10^{15}$ eV is likely of astrophysical origin, not due to hadroproduction thresholds.

ABSTRACT

This paper examines the transverse momentum spectra of baryons in the multi-particle production at modern colliders using Quark-Gluon String Model (QGSM). It discusses 1) the difference in Λ^0 hyperon spectra at antiproton-proton vs. proton-proton reactions; 2) the growth of average transverse momenta of Λhyperon with proton-proton collision energies and 3) the dependence of average p_t on the masses of mesons and baryons at the LHC energy 7 TeV. This analysis of baryon spectra led to the following conclusions. First, the fragmentation of antidiquark-diquark side of the pomeron diagram makes the major contribution to baryon production spectra in the asymmetric antiproton-proton reaction. Second, the average p_t's of hyperons steadily grow with energy on the range from 53 GeV to 7 TeV. Since no dramatic changes were seen in the characteristics of baryon production, the hadroproduction processes do not cause the "knee" in the cosmic ray proton spectra at the energies between Tevatron and LHC.

Motivation & Objective

• Examine the energy dependence of average transverse momentum ($p_t$) of $\Lambda^0$ hyperons in $p$-$p$ and $\bar{p}$-$p$ collisions from ISR to LHC.
• Assess whether baryon production dynamics undergo a significant change at LHC energies, particularly in relation to the cosmic ray proton spectrum 'knee' at $\sim 4 \times 10^{15}$ eV.
• Use the Quark-Gluon String Model (QGSM) to interpret and predict $p_t$ spectra across energy scales.
• Compare $p_t$ spectra between $p$-$p$ and $\bar{p}$-$p$ reactions to probe asymmetries in multi-particle production.
• Provide a foundation for improving Monte Carlo event generators (e.g., Pythia, LUND) by incorporating energy-dependent $p_t$ trends.

Proposed method

• Apply the Quark-Gluon String Model (QGSM) to describe baryon production via pomeron exchange, including diquark-antidiquark fragmentation into hyperons.
• Fit transverse momentum distributions $dN/dp_t$ with exponential functions $\exp(-B p_t)$ to extract slope parameters $B$ across energies.
• Compare $p_t$ spectra from $p$-$p$ (ISR, STAR, ALICE, ATLAS, CMS) and $\bar{p}$-$p$ (UA1, UA5, CDF) experiments across $\sqrt{s} = 53$ GeV to 7 TeV.
• Calculate average $p_t$ values from fitted spectra, focusing on the $p_t$ range 0.3–8 GeV/c where the slope has the strongest impact.
• Model the energy dependence of $p_t$ using the QGSM framework, accounting for $x_F$ dependence and multi-pomeron exchange contributions.
• Extend analysis to heavy baryons (B-mesons) using QGSM fits to LHCb data at $\sqrt{s} = 7$ TeV to assess mass dependence of $p_t$.

Experimental results

Research questions

• RQ1 How does the average transverse momentum ($p_t$) of $\Lambda^0$ hyperons evolve with center-of-mass energy from $\sqrt{s} = 53$ GeV to 7 TeV in $p$-$p$ collisions?
• RQ2 What explains the observed difference in $p_t$ spectra between $p$-$p$ and $\bar{p}$-$p$ collisions at similar energies, particularly in the context of QGSM?
• RQ3 Does baryon production at LHC energies ($\sqrt{s} = 7$ TeV) exhibit a dramatic change in dynamics that could explain the 'knee' in the cosmic ray proton spectrum at $\sim 4 \times 10^{15}$ eV?
• RQ4 Can the QGSM framework explain the energy dependence of $p_t$ spectra, particularly the hardening of spectra with increasing $\sqrt{s}$?
• RQ5 Is the observed $p_t$ growth consistent with multi-pomeron exchange effects and $x_F$ dependence in the QGSM model?

Key findings

• The average transverse momentum ($p_t$) of $\Lambda^0$ hyperons increases steadily with center-of-mass energy from $\sqrt{s} = 53$ GeV to 7 TeV, following a power-law dependence $\langle p_t \rangle \propto s^{0.05}$.
• The slope parameter $B$ of the exponential $p_t$ spectrum decreases from $B_0 = 4.6$ at ISR ($\sqrt{s} = 53$ GeV) to $B_0 = 2.1$ at LHC ($\sqrt{s} = 7$ TeV), indicating a hardening of the spectrum.
• QGSM explains the difference in $p_t$ spectra between $p$-$p$ and $\bar{p}$-$p$ collisions as due to asymmetric energy sharing in the pomeron diagram, with the diquark-antidiquark side in $\bar{p}$-$p$ absorbing more transverse energy.
• No significant change in baryon production dynamics is observed up to $\sqrt{s} = 7$ TeV, corresponding to a laboratory energy of $2.5 \times 10^{16}$ eV, suggesting the cosmic ray 'knee' is not due to hadroproduction thresholds.
• The average $p_t$ of B-mesons at $\sqrt{s} = 7$ TeV is estimated at 4.46 GeV/c, nearly equal to the B-meson mass, indicating strong transverse momentum generation in heavy-flavor hadroproduction.
• QGSM attributes the energy dependence of $p_t$ to residual $x_F$ dependence and short multi-pomeron chains, with energy conservation favoring harder spectra at higher $\sqrt{s}$.