Next-to-Leading Order Jet Physics with BlackHat

TL;DR

The paper demonstrates the viability of combining the BlackHat one-loop framework with SHERPA to produce next-to-leading order QCD predictions for vector-boson plus jet processes across Tevatron and LHC energies. It investigates scale choices and demonstrates how dynamical scales reduce theoretical uncertainties and stabilize distribution shapes. Key contributions include the first NLO results for Z+3-jet production in hadron collisions, a detailed study of W boson polarization at high transverse momentum, and an analysis of jet emission into rapidity gaps relevant for VBF-like searches. The work also documents improvements in agreement with data (where available) and outlines a path toward even more complex final states, including W+4 jets at NLO, facilitated by a public BlackHat release.

Abstract

We present several results obtained using the BlackHat next-to-leading order QCD program library, in conjunction with SHERPA. In particular, we present distributions for vector boson plus 1,2,3-jet production at the Tevatron and at the asymptotic running energy of the Large Hadron Collider, including new Z+3-jet distributions. The Z+2-jet predictions for the second-jet P_T distribution are compared to CDF data. We present the jet-emission probability at NLO in W+2-jet events at the LHC, where the tagging jets are taken to be the ones furthest apart in pseudorapidity. We analyze further the large left-handed W polarization, identified in our previous study, for W bosons produced at high P_T at the LHC.

Figures (9)

• Figure 1: LO and NLO predictions for the second jet $E_T$ distribution in $W+3$ jet production at the LHC. The only difference between the left and right panels is the scale choice: $\mu = E_T^W$ on the left and $\mu = \hat{H}_T$ on the right. The former choice is clearly problematic and should not be used in phenomenological studies. The bottom panels show the LO and NLO predictions, varied by a factor of two around the central scale, and divided by the NLO value at the central scale.
• Figure 2: The NLO $P_T$ distribution of the third jet in $Z\,\!+\,3$-jet production at the Tevatron. For the left panel the scale choice $\mu = E_T^Z$ is used, and for the right panel $\mu = \hat{H}_T/2$. Although the two NLO results are compatible, the LO results have large shape differences, illustrating that $\mu=\hat{H}_T/2$ is a better choice than $\mu = E_T^Z$ at the Tevatron as well. The lepton and jet cuts match the CDF ones ZCDF.
• Figure 3: Two distinct $W\,\!+\,3$ jet configurations with rather different values for the $W$ transverse energy. In configuration (a) an energetic $W$ balances the energy of the jets, while in (b) the $W$ is relatively soft. Configuration (b) generally dominates over (a) when the jet transverse energies get large.
• Figure 4: The scale dependence of the cross section for $Z\,\!+\,1,2,3$-jet production at the Tevatron, for the anti-$k_T$ jet algorithm using a leading-color approximation with $n_f$ terms, as a function of the common renormalization and factorization scale $\mu$, with $\mu_0 = M_Z$. The bottom panel shows the $K$ factors, or ratios between NLO and LO results, for the three cases.
• Figure 5: The second-jet $P_T$ distribution for $Z+2$ jets at LO and NLO compared against CDF data ZCDF.