Resummation effects in the forward production ofZ0+jetat the LHC

Abstract

We calculate several differential cross sections for Z0 and high-pT jet production in the forward rapidity region at the LHC using the hybrid High Energy Factorization. We test various unintegrated gluon distributions involving subleading BFKL effects (such as kinematic constraint, running strong coupling and DGLAP correction) and compare the results with experimental data obtained by the LHCb experiment. We find that the hard scale dependence of unintegrated gluon distributions, which effectively resums the Sudakov-type logarithms on the top of the resummation of the small x logarithms, is essential to describe the normalized azimuthal decorrelations between the Z0-boson and the jet. MotivationThe Large Hadron Collider opens an opportunity to explore kinematic regions where particles produced in high-energy collisions posses large transverse momenta and rapidities. The production of electroweak bosons and jets is a vital part of tests of Standard Model as well as searches of physics beyond the Standard Model. Furthermore it has been recognized in [1] that by studies of associated production of electroweak bosons and jets may provide a new insight into the transverse partonic structure of hadrons at small x, where x is the momentum fraction of the hadron taken by a parton participating in the hard collision. Furthermore, such a final state, being a combination of colorful and colorless particles, gives the opportunity for particularly interesting investigations complementary to results obtained in studies of pure jet final states [2][3][4][5][6] and Drell-Yan pairs [7]. In particular the final state rescatterings due to soft color exchanges should have less impact on the properties of produced final state as compared to pure jet final states. This work is motivated by a recent LHCb measurement [8] at √ s = 7 TeV of the process1 in the forward direction within the pseudorapidity range 2.0 < η < 4.5. The final-state muon and anti-muon were required to have transverse momenta p T µ > 20 GeV while the leading jet was considered with two different cuts: p T j > 10 GeV and p T j > 20 GeV. The jets were reconstructed using the anti-k T algorithm with radius R = 0.5 and they were required to be separated from muon tracks on the φ − η plane (azimuthal angle-pesudorapidity) by a distance R = 0.4. The muon-pair was required to have an invariant mass within the range 60 GeV < M µµ < 120 GeV. The rapidity constraint assures that in the partonic picture of the process, one of the initial state partons carries a rather small fraction x A of the corresponding hadron momentum p A , while the other must have a fraction x B x A (c.f. Fig. 1). In order to describe the process perturbatively, one definitely needs to go beyond the pure collinear factorization and support the calculation by a resummations. In the modern advanced approaches this is achieved by parton showers and hadronization as implemented for example in Pythia [9]. In the present work we consider another approach, namely a resummation of logarithms of ln(1/x ...

Saturation effects in forward-forward dijet production inp+Pbcollisions

“…The corresponding amplitudes were computed in [12] and cross-checked independently in [33,34], using different methods. The expressions are given in Appendix A.…”

“…For convenience, we collect here the matrix elements for the 2 → 2 partonic processes, computed in [12,33,34], and expressed in terms of the transverse momentum and rapidity of the final the state partons. Parametrizing the final state momenta as…”

QCD and jets at hadron colliders

“…The calculations can be performed with the public codes of Refs. [296,298,299]. Here, one can use a range of unintegrated gluons fitted to DIS data, like those obtained in Refs.…”

TMD gluon distributions for multiparton processes

“…The complete set of evolution equations in the linear regime can be also derived through the projector method [46], see [47] for a recent approach. There have been many calculations attacking various processes where the usage of unintegrated parton distributions is important, see for example [48][49][50][51][52][53][54][55][56][57][58][59] where mostly forward jet observables in hadroproduction were addressed. These calculations, however, use universal unintegrated gluon distributions, the same for any color flow.…”