Quark jet rates and quark-gluon discrimination in multijet final states

We estimate the number of quark jets in QCD multijet final states at hadron colliders. In the estimation, we develop the calculation of jet rates into that of quark jet rates. From the calculation, we estimate the improvement on the signal-to-background ratio for a signal semianalytically by applying quark-gluon discrimination, where the signal predicts many quark jets. We introduce a variable related to jet flavors in multijet final states and propose a data-driven method using the variable. As with the semianalytical result, the improvements in the signal-to-background ratio using the variable in Monte Carlo analysis are estimated.

Figure 1

Schematic illustration of initial- and final-state radiation. The central blob shows a hard process, $p$ is the transverse momentum for a final state, $z$ is the energy fraction for the final state, and $x$ and $x^{\prime }$ are the momentum fractions for an initial state and its parent parton.

Figure 2

The $x$ dependence of the coefficients in Eqs. (46)–(48). The vertical axis shows $x{p}_{\mathrm{beam}}$ which is the energy of an initial state. CTEQ6L1 is used in the calculations.

Figure 3

Expected values of the number of quark jets for events in which $N_{\text{jets}}$ jets are contained. The results for $gg\to gg$ (left), $gu\to gu$ (center) and $uu\to uu$ (right) are shown. In the calculation, $p_0=50\mathrm{GeV}$ and $R=0.4$ are used.

Figure 4

Jet $p_T$ dependence on gluon efficiencies for each quark efficiency (left panel). Dependence of the physics scale ${\mathrm{\Lambda }}_{\mathrm{new}}$ on the improvement factor ${\epsilon }_S/{\epsilon }_B$ for each $N_{\text{jets}}$ category (right panel).

Figure 5

The black and red curves show the distribution of $Q_i$ for the background and signal, where $H_T>2\mathrm{TeV}$, $N_{\text{jets}}\ge 8$ are imposed. We use the BDT outputs used in Sec. 3b as $Q_i$, which is trained such that the quark and gluon jet samples are assigned to 1 and 0.

Figure 6

The distribution of $d$ for the QCD multijets (left) and the signal (right) for each $N_{\text{jets}}$ category. The signal is set to $M_X=2\mathrm{TeV}$ and $n_X=5$.

Figure 7

The remaining rates of the number of events after imposing the $d$ cut for each $H_T$ bin. The left and right panels show the results for the QCD multijets and the signal at $N_{\text{jets}}\ge 6$. The signal parameters are $M_X=2\mathrm{TeV}$ and $n_X=5$. The dotted curves show an example of interpolation curves which are fitted by using data in a control region, i.e., $H_T<4\mathrm{TeV}$.

Figure 8

$M_X$ dependence on the efficiency ratio. We can see how the ratio changes as we increase the lower bound of $N_{\text{jets}}$ from 3 to 10, and $n_X$ from 2 (leftmost) to 5 (rightmost).

Figure 9

Same as Fig. 8, with the initial states $u\overline{u}$.

Figure 10

Matrix element correction to the number of quark jets with Catani-Krauss-Kuhn-Webber matching. The black curve is the result for $pp\to jj$ + parton showers. One and two partons are matched in the Born configuration and the results are shown by the red and blue curves, where we impose $H_T>2\mathrm{TeV}$ and set $\sqrt{s}=14\mathrm{TeV}$.

Figure 11

Same as Fig. 3, considering only the initial-state radiation.

Figure 12

Same as Fig. 3, considering only the final-state radiation.