Measurement of inclusive charged-particle b-jet production in pp and p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

A measurement of the inclusive b-jet production cross section is presented in pp and p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV, using data collected with the ALICE detector at the LHC. The jets were reconstructed in the central rapidity region $|\eta|<~0.5$ from charged particles using the anti-$k_{\rm T}$ algorithm with resolution parameter $R=0.4$. Identification of b jets exploits the long lifetime of b-hadrons, using the properties of secondary vertices and impact parameter distributions. The $p_{\rm T}$-differential inclusive production cross section of b jets, as well as the corresponding inclusive b-jet fraction, are reported for pp and p-Pb collisions in the jet transverse momentum range $10 \le p_{\text{T, ch jet}} \le 100$ GeV/$c$, together with the nuclear modification factor, $R_{\rm pPb}^{\text{b-jet}}$. The analysis thus extends the lower $p_{\rm T}$ limit of b-jet measurements at the LHC. The nuclear modification factor is found to be consistent with unity, indicating that the production of b jets in p--Pb at $\sqrt{s_{\rm NN}} = 5.02$ TeV is not affected by cold nuclear matter effects within the current precision. The measurements are well reproduced by POWHEG NLO pQCD calculations with PYTHIA fragmentation.

 

Submitted to: JHEP
e-Print: arXiv:2110.06104 | PDF | inSPIRE
CERN-EP-2021-205

Figure 1

Top left: The normalized impact parameter significance ($Sd_{xy}$) distribution for all tracks inside light-flavor, charm, and beauty jets as determined from PYTHIA~8 (Monash 2013 tune~) detector-level simulations. Top right: The distribution of the second largest impact parameter significance in the jet. Bottom left: The distribution of the third largest $Sd_{xy}$ in the jet. Bottom right: Distribution of $Sd_{xy}$ for data in \pp and \pPb collisions.

Figure 2

The logarithmic jet probability $-\ln(JP)$ distribution for light-flavor, charm, and beauty jets in \pp collisions at \five.

Figure 3

Distributions of the tagging discriminators used in the SV method, $SL_{xy}$ (left) and $\sigma_{\rm SV}$ (right), for b jets, c jets, and light-flavor jets as obtained from a MC simulation of the ALICE apparatus, using PYTHIA as an event generator.

Figure 5

Fit of the measured $-\ln(JP)$ discriminator distribution with a linear combination of b, c, and light-flavor jet templates for the untagged sample (left) and for the tagged sample (right).

Figure 6

The b-jet tagging efficiency extracted from the data-driven method using the IP algorithm in \pp and \pPb collisions.

Figure 7

Beauty-jet tagging efficiencies, as well as charm-jet, and light-flavor jet mistagging efficiencies for the SV method in pp (solid markers) and \pPb (open markers) collisions at $\snn=5.02$\,TeV, shown as a function of jet transverse momentum.

Figure 10

Purity of the b-jet candidates selected with the SV method when using the default tagging selection criteria. The purity was estimated with the data-driven template fit method (red points) and with the POWHEG-simulation based approach. The POWHEG scale variations accepted by the statistical analysis are colored green, the rejected ones are gray. Results for \pp and \pPb are shown in the left and right panel, respectively.

Figure 11

Comparison of the \pT differential production cross section of charged-particle anti-$k_{\rm T}$ $R=0.4$ b jets measured in pp and \pPb collisions at $\snn=5.02$\,TeV using the IP and SV methods. Systematic and statistical uncertainties are shown as boxes and error bars respectively. The additional common normalization uncertainty due to luminosity is denoted $\sigma_{\mathcal{L}}^{\rm Sys}$ and it is quoted separately.

Figure 12

Top panels: The combined differential production cross section of charged-particle anti-$k_{\rm T}$ $R=0.4$ b jets measured in \pp (left) and \pPb (right) collisions at $\snn=5.02$\,TeV. The data are compared with a NLO pQCD prediction by the POWHEG dijet tune with PYTHIA~8 fragmentation~. Systematic and statistical uncertainties are shown as boxes and error bars, respectively. The additional common normalization uncertainty due to luminosity, $\sigma_{\mathcal{L}}^{\rm Sys}$, is quoted separately. Bottom panels: Ratio of the theory calculations to the data.

Figure 13

The b-jet fraction in pp collisions at \five (left) and \pPb collisions at \fivenn (right), compared with POWHEG NLO pQCD calculations with PYTHIA~8 fragmentation.

Figure 14

Left: The nuclear modification factor \RpPbBjet of the inclusive charged-particle anti-$k_{\rm T}$ $R=0.4$ b jets as a function of \pt from the IP and SV method. Right: The nuclear modification factor \RpPbBjet obtained from combining the IP and SV method results as a function of \pTchjet compared with the calculation by the POWHEG dijet tune with the PYTHIA~8 fragmentation~ Systematic and statistical uncertainties are shown as boxes and error bars, respectively. There is an additional normalization uncertainty of $4.37\%$ due to luminosity, which is quoted separately.

Figure 15

The nuclear modification factor \RpPbBjet for charged-particle b jets measured by the ALICE experiment, compared with the b-jet measurement from the CMS experiment~. The CMS measurement represents $R=0.3$ fully reconstructed b jets within $-2.5

Figure 16

The nuclear modification factor of b jets compared to that of inclusive jets from Ref.~ There is a global normalization uncertainty of $4.37\%$ on the \RpPbBjet data from luminosity calculation, and $11.6\%$ on the inclusive-jet \RpPb data due to luminosity calculation and the scaling of the \pp reference.