Probe of unparticles at the LHC in exclusive two lepton and two photon production via photon-photon fusion

TL;DR

This study investigates unparticle contributions to exclusive LHC processes mediated by photon-photon fusion, focusing on $pp\to p\ell^-\ell^+p$ and $pp\to p\gamma\gamma p$ as clean channels with forward detectors. Using the Equivalent Photon Approximation, the authors compute the effective $\gamma\gamma$ luminosity within three forward-acceptance ranges $0.0015<ξ<0.15$, $0.0015<ξ<0.5$, and $0.1<ξ<0.5$, and quantify SM plus unparticle effects for scalar and tensor operators with scale dimension $d_U$ and scale $\Lambda_U$, including interference patterns in the tensor case. They derive 95% C.L. limits on unparticle couplings (e.g., $\kappa$, $\lambda_S$, $\lambda'_2$) as functions of $d_U$ and $\Lambda_U$ using Poisson statistics with optimized $p_T$ cuts and a $\chi^2$ angular-distribution method, across luminosities up to 200 fb$^{-1}$ and multiple detector acceptances. The results show that the $\ell^-\ell^+$ channel is particularly sensitive to scalar unparticles near $d_U\approx1$, while the $\gamma\gamma$ channel enhances sensitivity to tensor couplings at higher energies; overall, these exclusive channels provide complementary, background-suppressed probes of unparticle physics that are highly relevant for future $\gamma\gamma$ colliders as well.

Abstract

The exclusive production pp->pXp is known to be one of the most clean channels at the LHC. We investigate the potential of processes pp -> p l-l+ p and pp-> p gamma gamma p to probe scalar and tensor unparticles by considering three different forward detector acceptances; 0.0015<xi<0.15, 0.0015<xi<0.5 and 0.1<xi<0.5. We obtain 95% confidence level sensitivity limits on the unparticle couplings for various integrated luminosities.

Figures (17)

• Figure 1: Effective $\gamma\gamma$ luminosity as a function of the invariant mass of the two photon system. Figure on the left shows effective luminosity for forward detector acceptances $0.0015<\xi<0.15$ and $0.0015<\xi<0.5$. Figure on the right represents the cases with and without a cut on transverse momentum of the photon pair $|\vec{q}_{1t}+\vec{q}_{2t}|<30$MeV. In the right panel, we do not consider any acceptance i.e., $\xi$ is taken to be in the interval $0<\xi<1-m_p/E$ where $m_p$ is the mass and $E$ is the energy of the incoming proton.
• Figure 2: Tree-level Feynman diagrams for the subprocess $\gamma\gamma \to \ell^-\ell^+$.
• Figure 3: Cross section of $pp\to p\ell^-\ell^+p$ as a function of the transverse momentum cut on the final leptons. Solid line is for the SM and dotted lines $d_u=1.1$ and $d_u=3.001$ include scalar and tensor unparticle contributions respectively. Scalar unparticle couplings are taken to be $\kappa=\lambda_S=1$ and tensor unparticle couplings are taken to be $\lambda_2^\prime=1$ and $\lambda_2=10^3$.
• Figure 4: 95% C.L sensitivity of $pp\to p\ell^-\ell^+p$ to the product of scalar unparticle couplings $\kappa\lambda_S$ as a function of integrated LHC luminosity for an acceptance of $0.0015<\xi<0.5$. Various values of the scale dimension are stated on the figures. We impose the cuts; $|\vec{q}_{1t}+\vec{q}_{2t}|<30$MeV, $|\eta|<2.5$ and $p_t>460$GeV. $\Lambda_U$ is taken to be 3 TeV.
• Figure 5: 95% C.L sensitivity of $pp\to p\ell^-\ell^+p$ to the product of scalar unparticle couplings $\kappa\lambda_S$ as a function of integrated LHC luminosity for an acceptance of $0.0015<\xi<0.15$. Various values of the scale dimension are stated on the figures. We impose the cuts; $|\vec{q}_{1t}+\vec{q}_{2t}|<30$MeV, $|\eta|<2.5$ and $p_t>420$GeV. $\Lambda_U$ is taken to be 3 TeV.