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Measurement of electrons from heavy-flavour hadron decays as a function of multiplicity in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

The multiplicity dependence of electron production from heavy-flavour hadron decays as a function of transverse momentum was measured in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV using the ALICE detector at the LHC. The measurement was performed in the centre-of-mass rapidity interval $-1.07 <~ y_{\rm cms} <~ 0.14$ and transverse momentum interval 2 $<~ p_{\rm T} <~$ 16 GeV/$c$. The multiplicity dependence of the production of electrons from heavy-flavour hadron decays was studied by comparing the $p_{\rm T}$ spectra measured for different multiplicity classes with those measured in pp collisions ($Q_{\rm pPb}$) and in peripheral p-Pb collisions ($Q_{\rm CP}$). The $Q_{\rm pPb}$ results obtained are consistent with unity within uncertainties in the measured $p_{\rm T}$ interval and event classes. This indicates that heavy-flavour decay electron production is consistent with binary scaling and independent of the geometry of the collision system. Additionally, the results suggest that cold nuclear matter effects are negligible within uncertainties, in the production of heavy-flavour decay electrons at midrapidity in p-Pb collisions.

 

Submitted to: JHEP
e-Print: arXiv:1910.14399 | PDF | inSPIRE
CERN-EP-2019-250

Figures

Figure 1

Example of a trigger turn-on curve for the multiplicity class 0-20\%. Each scaling factor was obtained by fitting a constant to the plateau region (dashed lines) of the distribution. The resulting values are summarised in Tab.~\ref{tab:rej_factor}.
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Figure 2

The measured d$E$/d$x$ in the TPC expressed as a standard deviation from the expected energy loss of electrons, normalised by the energy-loss resolution ($\sigma_{\rm TPC}$) for $2 < \pt < 2.5$ \GeVc. The various curves are the different fit function results for the different peaks of the distribution. A Gaussian distribution, centered around zero, describes the electron candidates, and the pions and protons are the curves around $n_{\sigma}^{\rm TPC} < -4$ (Landau distribution multiplied by an Exponential distribution) and $n_{\sigma}^{\rm TPC} < -8$ (Gaussian distribution), respectively.
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Figure 3

$E/p$ distribution for $8 < \pt < 10$ \GeVc for the lower EMCal threshold triggered events (left) and for $12 < \pt < 14$ \GeVc for the higher EMCal threshold triggered events (right). The distributions are shown for electron candidates selected by the TPC ($-1 < n_{\sigma}^{\rm TPC} < 3$) (solid symbols) and for hadron candidates (open symbols) selected by the TPC $n_{\sigma}^{\rm TPC} < -3.5$.
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Figure 4

The \pt-differential invariant cross section of electrons from heavy-flavour hadron decays in p--Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV. The statistical uncertainties are indicated for both spectra by error bars and the systematic uncertainties are shown as boxes. The square markers show the published result for 0.5 $< $ \pt $< $ 8 \GeVc~, and the solid triangle and circle markers show the measurement up to \pt = 20 \GeVc using the EMCal trigger.
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Figure 5

The \pt-differential invariant cross section of electrons from heavy-flavour hadron decays in several charged-particle multiplicity classes in p--Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV. The statistical uncertainty of each spectrum is indicated by error bars and the systematic uncertainties are indicated by boxes.
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Figure 6

Nuclear modification factor, \RpPb, of electrons from heavy-flavour hadron decays as a function of transverse momentum for minimum-bias p--Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV. The vertical bars represent the statistical uncertainties, and the boxes indicate the systematic uncertainties. The systematic uncertainty from the normalisation, common to all points, is shown as a solid box at high \pt at \RpPb = 1. The published results~ are updated using the heavy-flavour hadron decays measurement obtained by ALICE in pp collisions at $\sqrt{s}$ = 5.02 TeV~. The points above 8 \GeVc are updated and extended using the EMCal trigger. The results are compared with theoretical models~, as described in the text.
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Figure 7

Nuclear modification factors $Q_{\rm pPb}$ as a function of \pt in the 0-20\%, 20-40\%, 40-60\%, and 60-100\% multiplicity classes selected with the ZNA estimator in \pPb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV. The different panels of the figure are for different multiplicity classes. The vertical error bars and the empty boxes represent the statistical and systematic uncertainties, respectively. The solid boxes at high \pt at $Q_{\rm pPb}$ = 1 represent the normalisation uncertainties. The results are compared with the PHENIX results on electrons from heavy-flavour hadron decays~ in d$+$Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV and with ALICE charged particle results~ in \pPb collisions at $\sqrt{s_{\rm NN}}$ = 5.02~TeV.
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Figure 8

$Q_{\rm cp}$ of electrons from heavy-flavour hadron decays in 0-20\%, 20-40\% and 40-60\% multiplicity classes in p--Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV. The vertical error bars and the empty boxes represent the statistical and systematic uncertainties, respectively. The solid boxes at high \pt at $Q_{\rm cp}$ = 1 represent the normalisation uncertainties. The results are compared to ALICE results on charged particles in p--Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV ~.
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