Z plus jets production via double parton scattering in pA collisions at the LHC

Eur. Phys. J. C (2020) 80:762 https://doi.org/10.1140/epjc/s10052-020-8336-5 Regular Article - Theoretical Physics Z plus jets production via double parton scattering in p A collisions at the LHC Boris Blok1,a , Federico Alberto Ceccopieri1,2,b 1 Department of Physics, Technion, Israel Institute of Technology, 32000 Haifa, Israel 2 IFPA, Université de Liège, 4000 Liège, Belgium Received: 16 April 2020 / Accepted: 9 August 2020 © The Author(s) 2020 Abstract We present results on Z j j production via double parton scattering in p A collisions at the LHC. We perform the analysis at leading and next-leading order accuracy with different sets of cuts on jet transverse momenta and accounting for the single parton scattering background. By exploiting the experimental capability to measure the centrality dependence of the cross section, we discuss the feasibility of DPS observation in already collected data at the LHC and in future runs. impact parameter than single parton scattering (SPS) and DPS1 contributions. Namely while the latter contributions are proportional to the nuclear thickness function T (B), B being the p A impact parameter, the DPS2 contribution is proportional to the square of T (B). Therefore the cross section for producing a given final state can be schematically written as:   2 d 2σ p A LT D P S1 T (B) D P S2  T (B) = σ + σ , + σ pA pA pA d2 B A d 2 B T 2 (B) (1) 1 Introduction The study of multiple parton interaction (MPI) and in particular of hard double parton scattering (DPS) reactions in p A collisions is important for our understanding of MPI in pp collisions. Significant progresses were achieved in study of double parton scattering in proton–nucleus collisions for a variety of final states [1–8] and implemented in PYTHIA Monte Carlo simulation [9]. The theory of DPS in p A collisions was developed in [10,11], where it was shown that there are two DPS contributions at work in such a case. First, there is the so-called DPS1 contribution, in which two partons from the incoming nucleon interact with two partons in the target nucleon in the nucleus and which is formally identical to DPS in pp collisions [12–23]. Then there is a new type of contribution, which we refer to as DPS2, in which two partons from the incoming nucleon interact with two partons each of them belonging to the distinct nucleons in the target nucleus located at the same impact parameter. Recently a new method was suggested [24] which could allow the observation of DPS2 in p A collisions. It was pointed out that the DPS2 has a different dependence on a e-mail: (corresponding author) b e-mail: 0123456789().: V,-vol where T (B) is normalized to the atomic number A of the nucleus. This observation gives the possibility to distinguish the DPS2 contribution in p A collisions from both the leading twist (LT) SPS and DPS1 contributions that are instead linear in T (B). This strategy has been adopted in our recent papers where we have analyzed the associated production of electroweak W boson and jets [25] and multijet production [26] via DPS in p A collisions. There we have shown that, exploiting the experimental capability of measuring the centrality dependence of the cross section [27–29], one can separate the DPS2 mechanism exploiting its different dependence on T (B), as it appears from Eq. (1). We found that the procedure can be successfully carried on for those final states by using the data already recorded in 2016 p A runs at the LHC. In this study we shall extend those results to the Z j j final state. We will show that, even in this case, by applying the very same technique, one can separate the DPS2 contribution from the DPS1+SPS background, despite the lower event rate associated to Z production, as compared, for example, to the ones for final states analysed in Refs. [25,26]. We present our results at leading order (LO) and nextto-leading order (NLO) accuracy. In the former approximation we find that, by using symmetric cuts on jet transverse momenta, it will be possible to observe the DPS2 contribution in p A data already collected at LHC. To NLO accuracy 123 762 Page 2 of 8 Eur. Phys. J. C we were able to study only the case of asymmetric cuts, cut − p cut ≥ 10 GeV, for the difference in the transi.e. p1T 2T verse momentum cuts of the leading and subleading jet. We adopted this prescription in order to tackle reliability issues inherent to the dijet NLO calculation as the difference of jet transverse momentum threshold is lowered. In the limit of vanishing transverse momentum difference, i.e. in the symmetric cut limit, the predictivity of the theory is recovered by performing an all order soft gluon resummation which is, however, beyond the scope of the current paper. The analysis within asymmetric cuts shows that NLO corrections lead to a slightly stronger DPS2 signal with respect to LO ones. However the statistical significance of the DPS2 signal decreases as a result of the reduced dijet rates obtained with asymmetric cuts choice for which we were able to determine NLO corrections. In such a case we may need higher statistics for detailed analysis of Z j j final state, although the signal can be appreciated already within the available p A with the lowest jet transverse momentum threshold. The paper is organized as follows. In Sect. 2 we briefly review the theoretical formalism at the base of our calculations. In Sect. 3 we present our results at leading order accuracy with symmetric cuts on the jet transverse momenta. In Sect. 4 we present our results at leading and next-to-leading order accuracy with asymmetric cuts on the jet transverse momenta. We summarize our findings in Sect. 5. 2 Calculation In this paper we consider the production of Z boson plus dijet in proton-lead collisions: p Pb → Z + 2 jets + X, where the Z decays leptonically and at least two jets are found in the final state. The corresponding DPS cross section (with C = Z and D = j j) is written to leading order accuracy as [10,11,25,26]:   dσ DC PDS j i k l = σe−1 f f f p (x 1 ) f p (x 2 ) f N (x 3 ) f N (x 4 ) d1 d2 i, j,k,l N = p,n × + D  d σ̂ikC d σ̂ jl dC d D   d 2 B TN (B) j i, j,k,l N3 ,N4 = p,n d σ̂ikC d σ̂ jl dC d D D × f pi (x1 ) f p (x2 ) f Nk 3 (x3 ) f Nl 4 (x4 )  d 2 B TN3 (B)TN4 (B). (2) The nuclear thickness function TN (B) appearing in Eq. (2), is obtained integrating the proton and neutron den( p,n) sities ρ0 (B, z) in the nucleus over the longitudinal com- 123 (2020) 80:762 ponent z. Following Refs. [25,26,30], for the 208 Pb nucleus, the density of proton and neutron is described by a WoodSaxon distribution whose parameters are fixed according to the analyses of Refs. [31,32]. The first term in Eq. (2) corresponds to the DPS1 mechanism, linear in the nuclear thickness function T A . It is calculated by assuming σe f f =18 mb, the average of experimental extracted value for similar final states [33,34] in DPS analyses in pp collisions. The se (...truncated)