[Paper Review] Charm hadron spectroscopy with ZEUS

Uri Karshon|arXiv (Cornell University)|Jul 1, 2004

Quantum Chromodynamics and Particle Interactions3 citations

TL;DR

This paper presents a search for excited charm mesons and a charm pentaquark state using 126.5 pb⁻¹ of ZEUS data from 1995–2000. Using $D^{*\pm}$ tagging via $D^{*+} \to D^0\pi_S^+$ and $D^0 \to K^-\pi^+$, the study confirms the $D_1^0(2420)$, $D_2^{*0}(2460)$, and $D_{s1}^\pm(2536)$ states, finds no evidence for the radially excited $D^{*\prime\pm}(2637)$, and sets a strict upper limit of <1% on the branching fraction for a charm pentaquark $\theta_c^0$ decaying to $D^{*\pm}p^\mp$ at 3.1 GeV, contradicting earlier H1 claims.

ABSTRACT

Neutral orbitally excited P-wave charm mesons have been reconstructed in the D*+-pi-+ final state and the charm-strange meson D+-s1(2536) was found in the D*+-K0s final state. A search for radially excited charm mesons in the D*+-pi+pi- final state has also been performed. A search for a charm pentaquark state near 3.1 GeV was made in the decay mode D*+-p-+. Using more than 40,000 reconstructed D* mesons, no resonance structure was observed.

Motivation & Objective

• To search for excited charm mesons, including radially excited $D^{*\prime\pm}$, via $D^{*\pm}\pi^{+}\pi^{-}$ decay modes.
• To search for the charm pentaquark $\theta_c^0$ in the $D^{*\pm}p^\mp$ final state, motivated by theoretical predictions and the H1 Collaboration's 2004 claim.
• To confirm the existence and properties of known P-wave charm states, such as $D_1^0(2420)$, $D_2^{*0}(2460)$, and $D_{s1}^\pm(2536)$, using high-statistics $D^{*\pm}$ samples.
• To set a stringent upper limit on the fraction of $D^{*\pm}$ mesons originating from a hypothetical charm pentaquark state at 3.1 GeV, resolving a discrepancy with H1 results.

Proposed method

• Reconstructed $D^{*\pm}$ mesons via the decay chain $D^{*+} \to D^0\pi_S^+ \to (K^-\pi^+)\pi_S^+$, using mass difference $\Delta M = M(K\pi\pi_S) - M(K\pi)$ to identify $D^{*\pm}$ candidates with high purity.
• Used unbinned likelihood fits to extract resonance signals, modeling signal shapes with Breit-Wigner distributions convoluted with Gaussian resolution functions and helicity amplitudes for $J^P = 1^+$ and $2^+$ states.
• Applied $V^0$-finding algorithms and $K^0_S \to \pi^+\pi^-$ vertex reconstruction to identify $K^0_S$ candidates for $D_{s1}^\pm(2536)$ reconstruction.
• Implemented proton identification using $dE/dx$ and momentum cuts in two ranges: low-$P$ ($P<1.35$ GeV) and high-$P$ ($P>2$ GeV), with optimized $dE/dx$ bands to suppress pion and kaon backgrounds.
• Performed systematic checks by varying $dE/dx$ thresholds, removing $D_1^0$ and $D_2^{*0}$ reflections, and matching H1 selection criteria to test signal robustness.
• Used Monte Carlo simulations to extrapolate to full phase space and derive a 95% confidence level upper limit on the product branching fraction $f(c\to D^{*\prime+}) \cdot B_{D^{*\prime+}\to D^{*+}\pi^+\pi^-} < 0.7\%$.

Experimental results

Research questions

• RQ1 Does the $D^{*\prime\pm}(2637)$ state, observed by DELPHI, appear in the $D^{*\pm}\pi^+\pi^-$ final state in ZEUS data?
• RQ2 Is there evidence for a narrow resonance in the $M(D^{*\pm}p^\mp)$ spectrum consistent with a charm pentaquark $\theta_c^0$ at 3.1 GeV?
• RQ3 What is the upper limit on the fraction of $D^{*\pm}$ mesons originating from a hypothetical $\theta_c^0$ state decaying to $D^{*\pm}p^\mp$?
• RQ4 Are the properties of known P-wave charm states, such as $D_1^0(2420)$ and $D_{s1}^\pm(2536)$, consistent with PDG values and theoretical expectations?

Key findings

• The $D_1^0(2420)$ and $D_2^{*0}(2460)$ states were observed with 526±65 and 203±60 reconstructed events, respectively, in the $D^{*\pm}\pi^{\mp}$ final state.
• A narrow enhancement at 2398.1±2.1 MeV (stat.) was observed in the $D^{*\pm}\pi^{\mp}\pi^{\mp}$ final state, but its significance is uncertain due to possible interference or statistical fluctuation.
• The $D_{s1}^\pm(2536)$ state was confirmed with 62.3±9.3 reconstructed events, with a mass of 2534.2±0.6±0.5 MeV, consistent with the PDG value.
• The helicity analysis of $D_{s1}^\pm(2536)$ yielded $R = -0.53±0.32$ (stat.) ±0.05 (syst.), consistent with $J^P = 1^+$ or $1^-$, though not definitively distinguishing between them.
• No evidence was found for the radially excited $D^{*\prime\pm}(2637)$ state in the $D^{*\pm}\pi^+\pi^-$ final state, with a 95% CL upper limit of $f(c\to D^{*\prime+}) \cdot B_{D^{*\prime+}\to D^{*+}\pi^+\pi^-} < 0.7\%$.
• The data exclude a contribution of more than 1% to the $D^{*\pm}$ rate from a charm pentaquark $\theta_c^0$ decaying to $D^{*\pm}p^\mp$ at 3.1 GeV, contradicting the H1 Collaboration's earlier claim.