[Paper Review] A note on the CDF high-p_t charged particle excess

Matteo Cacciari, Gavin P. Salam+1 more|arXiv (Cornell University)|Mar 17, 2010

Particle physics theoretical and experimental studies23 references3 citations

TL;DR

This paper investigates the CDF experiment's observed excess of high-pT charged particles, ruling out QCD factorization violation by comparing charged-particle and jet cross sections. It shows that for pt > 80 GeV, the two spectra become comparable, and data on particle momentum fractions within jets (showing only 0.1% of jets carry >90% momentum in one hadron) contradict the idea that a single hadron dominates high-pt jets, thus invalidating the factorization violation hypothesis. The excess remains unexplained but is unlikely due to new physics or experimental artifacts.

ABSTRACT

It has recently been pointed out that CDF data for the cross section of high-p_t charged particles show an excess of up to three orders of magnitude over QCD predictions, a feature tentatively ascribed to possible violations of factorisation. We observe that for p_t > 80 GeV the measured charged-particle cross sections become of the same order as jet cross sections. Combining this information with data on charged particle distributions within jets allows us to rule out the hypothesis that the CDF data could be interpreted in terms of QCD factorisation violation. We also comment on the difficulty of interpreting the excess in terms of new physics scenarios.

Motivation & Objective

• To assess whether the CDF high-pT charged particle excess can be explained by a violation of QCD factorization.
• To test the consistency of the excess with known QCD dynamics using inclusive jet and charged particle spectra from the same Tevatron data.
• To evaluate whether the excess could arise from exotic new physics, such as R-hadrons or long-lived particles.
• To determine whether the observed effect is likely due to experimental artifacts or misinterpretation of data.

Proposed method

• Compare the CDF inclusive charged-particle cross section with inclusive jet cross sections at high pt (>80 GeV), where both spectra become comparable in magnitude.
• Use jet data from CDF (using kt algorithm, R=0.5) and NLO QCD predictions (FastNLO, NLOJet++) to validate the jet spectrum.
• Analyze the momentum fraction distribution of charged particles within jets from CDF dijet events (fig. 3), focusing on the fraction carrying >90% of jet momentum.
• Compute the (n−1)th moment of the momentum fraction distribution C(x) to predict the ratio of charged-particle to jet spectra, with n ≈ 6 from the jet spectrum's pt−n fall-off.
• Assess the consistency of the excess with R-hadron or other exotic new physics scenarios, considering detection inefficiencies and missing transverse energy signatures.
• Suggest direct inspection of signal events for anomalies such as mismatched pT and energy deposition, time-of-flight delays, or late decays to distinguish real particles from fake tracks.

Experimental results

Research questions

• RQ1 Can the CDF high-pT charged particle excess be explained by a breakdown of QCD factorization in fragmentation functions?
• RQ2 Is the observed excess consistent with standard QCD predictions when comparing charged-particle and jet cross sections at high pt?
• RQ3 Do data on charged particle momentum fractions within jets rule out the hypothesis that a single hadron carries most of a high-pt jet’s momentum?
• RQ4 Could the excess be due to exotic new physics such as R-hadrons or long-lived particles with low curvature tracks?
• RQ5 Are there detectable signatures in event-level data that would distinguish a real high-pT track from a misreconstructed or fake track?

Key findings

• For pt > 80 GeV, the inclusive charged-particle cross section becomes comparable in magnitude to the inclusive jet cross section, indicating that high-pt jets are not dominated by single hadrons.
• CDF data on charged particle momentum fractions within jets show that only about 0.1% of jets contain a single hadron carrying more than 90% of the jet’s momentum, contradicting the idea that a single hadron accounts for the entire jet.
• The predicted ratio of charged-particle to jet spectra, derived from the (n−1)th moment of C(x), is approximately 0.006, consistent with Pythia and NLO QCD predictions.
• The hypothesis of QCD factorization violation is ruled out because it would require collinear splitting to cease at high parton energies, which contradicts observed jet substructure and momentum fraction distributions.
• Exotic new physics scenarios such as R-hadrons are considered unlikely due to strong constraints from missing transverse energy and jet spectrum distortions, though some loopholes may remain for light, neutral, or long-lived states.
• The authors conclude that the excess is not due to experimental artifacts or misinterpretation, and recommend direct inspection of signal events to identify potential new physics or data anomalies.