ATLAS search for new phenomena in dijet mass and angular distributions using pp collisions at $ \sqrt{s}=7 $TeV

Abstract

Mass and angular distributions of dijets produced in LHC proton-proton collisions at a centre-of-mass energy √ s = 7 TeV have been studied with the ATLAS detector using the full 2011 data set with an integrated luminosity of 4.8 fb −1 . Dijet masses up to ∼ 4.0 TeV have been probed. No resonance-like features have been observed in the dijet mass spectrum, and all angular distributions are consistent with the predictions of QCD. Exclusion limits on six hypotheses of new phenomena have been set at 95% CL in terms of mass or energy scale, as appropriate. These hypotheses include excited quarks below 2.83 TeV, colour octet scalars below 1.86 TeV, heavy W bosons below 1.68 TeV, string resonances below 3.61 TeV, quantum black holes with six extra space-time dimensions for quantum gravity scales below 4.11 TeV, and quark contact interactions below a compositeness scale of 7.6 TeV in a destructive interference scenario. The ATLAS collaboration 30At the CERN Large Hadron Collider (LHC), collisions with the largest momentum transfer typically result in final states with two jets of particles with high transverse momentum (p T ). The study of these events tests the Standard Model (SM) at the highest energies accessible at the LHC. At these energies, new particles could be produced [1,2], new interactions between particles could manifest themselves [3-6], or interactions resulting from the unification of SM with gravity could appear in the TeV range [7][8][9][10][11][12]. These collisions also probe the structure of the fundamental constituents of matter at the smallest distance scales allowing, for example, an experimental test of the size of quarks. The models for new phenomena (NP) tested in the current studies are described in section 9. The two jets emerging from the collision may be reconstructed to determine the twojet (dijet) invariant mass, m jj , and the scattering angular distribution with respect to the colliding beams of protons. The dominant Quantum Chromodynamics (QCD) interactions for this high-p T scattering regime are t-channel processes, leading to angular distributions that peak at small scattering angles. Different classes of new phenomena are expected to modify dijet mass distribution and the dijet angular distributions as a function of m jj , creating either a deviation from the QCD prediction above some threshold or an excess of events localised in mass (often referred to as a "bump" or "resonance"). Most models predict that the angular distribution of the NP signal would be more isotropic than that of QCD.Results from previous studies of dijet mass and angular distributions [13][14][15][16][17][18][19][20][21][22][23] were consistent with QCD predictions. The study reported in this paper is based on pp collisions at a centre-of-mass (CM) energy of 7 TeV produced at the LHC and measured by the ATLAS detector. The analysed data set corresponds to an integrated luminosity of 4.8 fb −1 collected in 2011 [24,25], a substantial increase over previously published ATLAS dijet analyses [22, 23].A d...

Three- and four-point correlators of excited bosonic twist fields

“…Indeed, from dijet searches for related resonances [39][40][41], since the cross-section is so suppressed we can infer that the bounds on such states will be much less than a TeV. This raises the intriguing prospect that the string scale could be just out of reach of the LHC, but the light stringy states could be hiding in plain sight.…”

“…They show that these amplitudes exhibit a universal behaviour independently of the specifics of the compactication, which gives their results a predictive power. The observed poles correspond to the exchanges of Regge excitations of the standard model gauge bosons, whose mass scales like the string mass, and this has enabled the experiments to now limit the string scale to be above 5.08 TeV [39][40][41].…”

Constraining flavoured contact interactions at the LHC

“…Furthermore, the flavour structure of contact interactions induced by New Physics is explored by precision flavour physics (mostly for flavour off-diagonal operators), so the collider bounds should also be for specific flavour indices, to allow comparison and combination with low-energy observations. However, the current LHC bounds are given for a subset of flavour-summed operators [3][4][5]. The aim of this paper is to make an approximate translation of the collider constraints onto individual, flavoured operators and to illustrate the limits of such a translation.…”

“…Two issues arise in attempting to apply the published contact interactions bounds to a different operator: the Lorentz and gauge structure of the operator affects the partonic cross section, and the flavour indices affect possible interferences with QCD, as well as controlling the probability of finding the initial state quarks in the proton. Let us mention that four-quark contact interactions have been searched for at the Tevatron in qq → dijets [6], and at the LHC in qq → dijets [3][4][5]. In both cases, the initial state q orq are first generation valence quarks, whereas the final state can be of any flavour, so the Tevatron and LHC can constrain different flavour structures.…”

“…Section 2 reviews the kinematical variables used by the experimental collaborations [3,4,6] to constrain contact interactions from the rapidity distribution of high energy dijets. These variables allow an approximate "factorisation" of the pp → dijets cross section into an integral over parton distribution functions (pdfs), multiplying a partonic cross section.…”