Forward Dihadron Research Articles

Signature of gluon saturation in forward di-hadron correlations at the Large Hadron Collider

We argue that gluon saturation effects may be visible in Large Hadron Collider (LHC) data. We look at the centrality dependence of the away-side peak of two-particle correlations measured in p-Pb collisions, using both ALICE data at midrapidity, and LHCb data at forward rapidity. Once the collective flow-like signal measured at the near-side peak is removed from the correlation function, the centrality dependence of the away-side peak turns out to be much stronger in the forward region than at midrapidity. We argue that this is a very specific prediction of the saturation framework, and we compare LHCb data to a state-of-the-art calculation within the Color Glass Condensate theory.

Consequences of high-xproton size fluctuations in small collision systems atsNN=200GeV

Recent measurements of jet production rates at large transverse momentum ($p_T$) in the collisions of small projectiles with large nuclei at RHIC and the LHC indicate that they have an unexpected relationship with estimates of the collision centrality. One compelling interpretation of the data is that it captures an $x_p$-dependent decrease in the average interaction strength of the nucleon in the projectile undergoing a hard scattering. A weakly interacting or "shrinking" nucleon in the projectile strikes fewer nucleons in the nucleus, resulting in a particular pattern of centrality-dependent modifications to high-$p_T$ processes. We describe a simple one-parameter geometric implementation of this picture within a modified Monte Carlo Glauber model tuned to $d$$+$Au jet data, and explore two of its major consequences. First, the model predicts a particular projectile-species dependence to the centrality dependence at high-$x_p$, opposite to that expected from an energy loss effect. Second, we find that some of the large centrality dependence observed for forward di-hadron production in $d$$+$Au collisions at RHIC may arise from the physics of the "shrinking" projectile nucleon, in addition to impact parameter-dependent shadowing or saturation effects at low nuclear-$x$. We conclude that analogous measurements in recently collected $p$$+$Au and $^3$He$+$Au collision data at RHIC can provide a unique test of these predictions.

The search for gluon saturation in pA collisions

In this talk, I reviewed the recent theoretical progress towards the exploration of gluon saturation phenomenon in pA collisions at both RHIC and the LHC. Two important pillars of this exploration are the single inclusive forward hadron productions and forward dihadron correlations, which have both been computed up to one-loop order within the small- x factorization formalism. Furthermore, detailed numerical study for the single inclusive hadron production at next-to-leading order provides us good description of the low transverse momentum data measured in d Au collisions at RHIC with significantly reduced uncertainty. In addition, the one-loop calculation for dihadron productions in pA collisions shows that there is a new type of large logarithm, which is known as the Sudakov factor, arising in the back-to-back correlation limit.

Gluon splitting in a shockwave

The study of azimuthal correlations in particle production at forward rapidities in proton–nucleus collisions provides direct information about high gluon density effects, like gluon saturation, in the nuclear wavefunction. In the kinematical conditions for proton–lead collisions at the LHC, the forward di-hadron production is dominated by partonic processes in which a gluon from the proton splits into a pair of gluons, while undergoing multiple scattering off the dense gluon system in the nucleus. We compute the corresponding cross-section using the Colour Glass Condensate effective theory, which enables us to include the effects of multiple scattering and gluon saturation in the leading logarithmic approximation at high energy. This opens the way towards systematic studies of angular correlations in two-gluon production, similar to previous studies for quark–gluon production in the literature. We consider in more detail two special kinematical limits: the “back-to-back correlation limit”, where the transverse momenta of the produced gluons are much larger than the nuclear saturation momentum, and the “double parton scattering limit”, where the two gluons are produced by a nearly collinear splitting occurring prior to the collision. We argue that saturation effects remain important even for relatively high transverse momenta (i.e. for nearly back-to-back configurations), leading to geometric scaling in the azimuthal distribution.

Forward dihadron correlations in the Gaussian approximation of JIMWLK

We compute forward dihadron azimuthal correlations in deuteron-gold collisions using a Gaussian approximation for the quadrupole operator. The double parton scattering contribution is found to be part of our dihadron calculation. We obtain a good description of the PHENIX data for the azimuthal-angle dependent away side peak and a relatively good estimate for the pedestal contribution.

Forward particle correlations in the color glass condensate

Multiparticle correlations, such as forward dihadron correlations in pA collisions, are an important probe of the strong color fields that dominate the initial stages of a heavy ion collision. We describe recent progress in understanding two-particle correlations in the dilute-dense system, e.g. at forward rapidity in deuteron-gold collisions. This requires evaluating higher point Wilson line correlators from the JIMWLK equation, which we find well described by a Gaussian approximation. We then calculate the dihadron correlation, including both the "elastic" and "inelastic" contributions, and show that our result includes the double parton scattering contribution.

Forward dihadron correlations in deuteron–gold collisions with a Gaussian approximation of JIMWLK

We compute dihadron correlations in forward deuteron–gold or proton–gold collisions. The running coupling BK equation is used to calculate the energy dependence of the dipole cross sections and extended to higher-point Wilson line correlators using a factorized Gaussian approximation. Unlike some earlier works we include both the “inelastic” and “elastic” contributions to the dihadron cross section. We show that the double parton scattering contribution is included in our calculation and obtain both an away side peak that roughly agrees with experimental observations and an estimate for the azimuthal angle-independent pedestal. We find that nonlinear effects for momenta close to the saturation scale are clearly visible in the away side peak structure.

Drell-Yan lepton-pair-hadron correlation inpAcollisions

In this paper, we numerically study the forward correlations between the lepton-pair and associated hadrons in Drell-Yan process in pA collisions. Using the present knowledge of the dipole gluon distribution from the modified Golec-Biernat-W\"{u}sthoff model and from the solution of the Balitsky-Kovchegov evolution equation, we are able to compute and predict the forward correlations between the lepton-pair and associated hadron in Drell-Yan process at RHIC and LHC. Similar to the forward dihadron correlation in d-Au collisions measured at RHIC, the Drell-Yan type correlation also implies a strong suppression of the away side hadron at forward rapidity due to the strong interaction between the forward quark from the projectile proton and the gluon density from the target nucleus. Another feature of this process is that the correlation contains a double-peak structure in the away side, which makes it a unique observable.

Azimuthal Correlations of Forward Dihadrons ind+AuCollisions at RHIC in the Color Glass Condensate

We present a good description of recent experimental data on forward dihadron azimuthal correlations measured in deuteron-gold collisions at the BNL Relativistic Heavy Ion Collider (RHIC), where monojet production has been observed. Our approach is based on the color glass condensate theory for the small-x degrees of freedom of the nuclear wave function, including the use of nonlinear evolution equations with running QCD coupling. Our analysis provides further evidence for the presence of saturation effects in RHIC data.