Search for microscopic black holes in pp collisions at sqrt(s) = 8 TeV

TL;DR

This CMS study searches for microscopic black holes and string balls produced in $pp$ collisions at $\sqrt{s}=8~\text{TeV}$ using $12.1~\text{fb}^{-1}$ of data. The analysis employs a data-driven, $S_T$-based approach across multiple object multiplicities to distinguish potential signals from QCD multijet backgrounds, with limits set via the CL$_s$ method. No excess is observed, yielding 95% CL exclusions of BH/ string-ball masses in the $4.3$–$6.2~\text{TeV}$ range for various benchmark models and providing model-independent cross-section limits down to about $0.2~\text{fb}$ at high $S_T$. These results significantly extend previous collider bounds and constrain a wide class of models predicting energetic, multiparticle final states at the LHC.

Abstract

A search for microscopic black holes and string balls is presented, based on a data sample of pp collisions at sqrt(s) = 8 TeV recorded by the CMS experiment at the Large Hadron Collider and corresponding to an integrated luminosity of 12 inverse femtobarns. No excess of events with energetic multiparticle final states, typical of black hole production or of similar new physics processes, is observed. Given the agreement of the observations with the expected standard model background, which is dominated by QCD multijet production, 95% confidence limits are set on the production of semiclassical or quantum black holes, or of string balls, corresponding to the exclusions of masses below 4.3 to 6.2 TeV, depending on model assumptions. In addition, model-independent limits are set on new physics processes resulting in energetic multiparticle final states.

Figures (7)

• Figure 1: Distribution of the scalar sum of transverse energy, $S_\mathrm{T}\xspace$, for events with multiplicity: (Left) $N = 2$ and (right) $N \geq 2$ objects (photons, electrons, muons, or jets) in the final state. Observed data are depicted as points with statistical error bars; the solid line with a shaded band is the multijet background prediction from $N=2$ fit and its systematic uncertainty. Coloured histograms represent the $\gamma+\text{jets}$ (orange), ${V}+\text{jets}$ (red), and ${t}\overline{{t}}$ (green) backgrounds. Also shown are the expected semiclassical black hole signals for three parameter sets of the BlackMax nonrotating semiclassical black hole model, as well as a quantum black hole model. Here, $M_\mathrm{BH}^\text{min}\xspace$ is the minimum black hole mass, $M_\mathrm{QBH}^{\min}$ is the minimum quantum black hole mass, ${M_\mathrm{D}}\xspace$ is the multidimensional Planck scale, and $n$ is the number of extra dimensions. The bottom panels in each plot show the pull distribution (defined as ($\text{data} - \text{background})/\sigma(\text{data} - \text{background}$)) based on combined statistical and systematic uncertainty (dominated by the latter). Note that the systematic uncertainty is fully correlated bin-to-bin. Also shown in the $N = 2$ plot, is the background optimization based on a fit to $N=3$ data (dotted line). The difference between the $N=2$ and $N=3$ background fits are covered by the systematic uncertainty band used in the analysis.
• Figure 2: Distribution of the scalar sum of transverse energy, $S_\mathrm{T}\xspace$, for events with multiplicity: (Top left) $N \geq 3$, (top right) $N \geq 4$, (bottom left) $N \geq 5$, and (bottom right) $N \geq 6$ objects (photons, electrons, muons, or jets) in the final state. Observed data are depicted as points with statistical error bars; the solid line with a shaded band is the multijet background prediction and its systematic uncertainty. Also shown are the expected semiclassical black hole signals for three parameter sets of the BlackMax nonrotating black hole model, as well as a quantum black hole signal of the qbh model. Here, $M_\mathrm{BH}^\text{min}\xspace$ is the minimum black hole mass, $M_\mathrm{QBH}^{\min}$ is the minimum quantum black hole mass, ${M_\mathrm{D}}\xspace$ is the multidimensional Planck scale, and $n$ is the number of extra dimensions. The bottom panels in each plot show the pull distribution (defined as ($\text{data} - \text{background})/\sigma(\text{data} - \text{background}$)) based on combined statistical and systematic uncertainty (dominated by the latter). Note that the systematic uncertainty is fully correlated bin-to-bin.
• Figure 3: Distribution of the scalar sum of transverse energy, $S_\mathrm{T}\xspace$, for events with multiplicity: (Top left) $N \geq 7$, (top right) $N \geq 8$, (bottom left) $N \geq 9$, and (bottom right) $N \geq 10$ objects (photons, electrons, muons, or jets) in the final state. Observed data are depicted as points with statistical error bars; the solid line with a shaded band is the multijet background prediction and its systematic uncertainty. Also shown are the expected semiclassical black hole signals for three parameter sets of the BlackMax nonrotating black hole model. Here, $M_\mathrm{BH}^\text{min}\xspace$ is the minimum black hole mass, ${M_\mathrm{D}}\xspace$ is the multidimensional Planck scale, and $n$ is the number of extra dimensions. The bottom panels in each plot show the pull distribution (defined as ($\text{data} - \text{background})/\sigma(\text{data} - \text{background}$)) based on combined statistical and systematic uncertainty (dominated by the latter). Note that the systematic uncertainty is fully correlated bin-to-bin.
• Figure 4: The 95% CL lower limits on the semiclassical black hole mass derived from the upper 95% CL limits on cross section times branching fraction as a function of the fundamental Planck scale ${M_\mathrm{D}}\xspace$, for various models. The areas below each curve are excluded by this search. Top left: BlackMax black hole models without the stable remnant. Top right and bottom row: Charybdis black hole models with or without the stable remnant. The number $n$ of extra dimensions is labelled accordingly.
• Figure 5: (Left) The cross section upper limits at 95% CL from the counting experiments optimized for various string ball parameter sets (solid lines) compared with predicted signal production cross section (dashed lines) as a function of minimum string ball mass. Here, ${M_\mathrm{D}}\xspace$ is the multidimensional Planck scale, $M_\mathrm{S}$ is the string scale, and $g_\mathrm{S} = 0.4$ is the string coupling. (Right) Lower quantum black hole mass limits at $95\%$ CL as functions of the fundamental Planck scale ${M_\mathrm{D}}\xspace$ for various qbh black hole models with a number $n$ of extra dimensions from one to six.