LHC Phenomenology for String Hunters

TL;DR

This work investigates TeV-scale open-string/D-brane extensions of the Standard Model, focusing on Regge recurrences and extra-dimensional KK and winding modes as LHC-accessible signatures. The authors compute open-string amplitudes for dijet and related processes, revealing a universal form factor $V(s,t,u)$ that produces $s$-channel Regge poles at $s=n M_s^2$ and identifying non-resonant KK/winding contributions that generate 4-fermion contact terms, notably enhancing dijet observables via the $R$ ratio. They show that, even in the absence of $s$-channel qq resonances, KK/Wind effects can yield sizable deviations from QCD, and that a nearby string resonance at $s=n M_s^2$ would serve as a smoking gun; combining dijet and $Z^0+$jet channels offers corroboration. The analysis predicts strong early-LHC signals for $M_s$ up to about $3.5$ TeV in multiple channels, with potential $6\sigma$ discoveries for $M_s\lesssim 3$ TeV after about a year of data at $\sqrt{s}=10$ TeV, thereby providing concrete, largely model-independent tests of TeV-scale strings.

Abstract

We consider extensions of the standard model based on open strings ending on D-branes, with gauge bosons due to strings attached to stacks of D-branes and chiral matter due to strings stretching between intersecting D-branes. Assuming that the fundamental string mass scale is in the TeV range and the theory is weakly coupled, we discuss possible signals of string physics at the Large Hadron Collider (LHC). In previous works, direct channel excitations of Regge recurrences in parton-parton scattering supplied the outstanding new signature. The present work considers the deviation from standard model expectations for the 4-fermion processes qq\to qq and qq' \to qq', in which the s-channel excitation of string resonances is absent. In this case, we find that Kaluza-Klein recurrences at masses somewhat less than the string scale generate effective 4-fermion contact terms which can significantly enhance the dijet R ratio above its QCD value of about 0.6. The simultaneous observation of a nearby resonant structure in the dijet mass spectrum would provide a "smoking gun" for TeV scale string theory. In this work, we also show that (1) for M_{string}<3.5 TeV, the rates for various topologies arising from the pp \to Z^0 + jet channel could deviate significantly from standard model predictions and (2) that the sizeable cross sections for Regge recurrences can allow a 6σdiscovery for string scales as large as 3 TeV after about 1 year of LHC operation at \sqrt{s} =10 TeV and \int L dt ~ 100 pb^{-1}.

Figures (3)

• Figure 1: For integrated luminosities of 1 fb$^{-1}$ (left panel) and 10 fb$^{-1}$ (right panel) the expected value (solid line) and statistical error (shaded region) of the dijet ratio of QCD in the CMS detector Esen is compared with LO QCD (dashed line), LO QCD + lowest massive string excitations (dot-dashed), and LO QCD + lowest massive string excitations + non-resonant string scattering in $qq \to qq$ (dot-solid). We have taken $M_{ab} = 3.5~{\rm TeV}$, $M_s = 5$ TeV, and $g^2_b/g_a^2 = 1/3$.
• Figure 2: For integrated luminosities of 1 fb$^{-1}$ (left panel) and 10 fb$^{-1}$ (right panel) the expected value (solid line) and statistical error (shaded region) of the dijet ratio of QCD in the CMS detector Esen is compared with LO QCD (dashed line), LO QCD + lowest massive string excitations (dot-dashed), and LO QCD + lowest massive string excitations + non-resonant string scattering in $qq \to qq$ and $qq' \to qq'$ (dot-solid), with all anomalous $U(1)$ masses set to zero (see text). We have taken $M_{ab} = \infty~{\rm TeV}$, $M_s = 5$ TeV, and $g^2_b/g_a^2 = 1/3$.
• Figure 3: $d\sigma/dM$ (units of fb/GeV) vs.$M$ (TeV) is plotted for the case of SM QCD background (dot-dashed line) and (first resonance) string signal + background (solid line), for $M_s = 2$ TeV (left) and $M_s = 3$ TeV (right).