Hopes are rising that a previously unknown particle has been discovered by the Large Hadron Collider, the particle accelerator that lies under the France-Switzerland border near Geneva. The LHC was the site of the discovery of the Higgs boson, for which the scientists who predicted its existence won the 2013 Nobel Prize in physics.
Data obtained from experiments conducted through last year indicates the possible existence of a new particle with roughly six times the mass of the Higgs boson. In tests this year — which began in May — the facility will try to determine whether or not the particle actually exists. While a number of theories have already emerged to explain the nature of this particle, its mere existence, if proven, could rewrite the existing standard theory of elementary particles.
The data suggesting the existence of the new particle was unveiled at a conference held in December 2015 by the European Organization for Nuclear Research, or CERN, which operates the LHC. Atlas and CMS, two international research groups that discovered the Higgs boson in 2012, each exhibited data suggesting that a particle with an energy — and thus a mass — of 750 gigaelectronvolts (GeV) had been born in experiments involving head-on collisions of protons accelerated to nearly the speed of light. The new particle immediately decayed, and two photons were observed shooting out from it. By measuring the energies of these two photons, researchers were able to calculate the mass of the underlying particle. This was the same method used to discover the Higgs boson.
A mass of 750GeV is considerably larger than that of any other elementary particle discovered so far, including the top quark (173GeV) and the Higgs boson (125GeV).
Extremely high statistical precision is needed in experiments looking for new particles with accelerators. This precision is measured in standard deviations, or sigma, for short. To claim that a new particle has been discovered, the observed results must have a certainty level of greater than five sigma, which means the probability of the results being a statistical fluke is around 1 in 3 million.
The Atlas experiment had a certainty level of just under 4 sigma, while that of the CMS experiment was around 3 sigma. But while each experiment on its own fell short of achieving the status of “discovery,” the fact that data showing the presence of a particle with virtually the same mass was discovered through two independently executed experiments has raised hopes that it does indeed exist.
On May 9, CERN announced it had restarted operation of the LHC for the first time in half a year. It will be in operation through early November and will compile an enormous volume of experimental data, increasing the number of proton collisions from 300 trillion last year to 2,500 trillion, roughly 8 times as many.
University of Tokyo professor Shoji Asai, a joint representative of the Atlas Japan group, is not placing any bets on the outcome. “To be honest, whether or not this will be confirmed as a new particle is a 50-50 proposition,” he said. “There is still a high likelihood of statistical scattering. Because sufficient data will have been accumulated within one month of the start of experiments, the conclusion of whether it is a new particle or not should be out in July or August.”
three categories of Papers on the new particle
1. the new particle is in the same category as the Higgs boson. The current standard theory is that there are 17 types of elementary particles, including quarks, neutrinos and the Higgs boson. There is a theory that expands on this to say that partner particles, known as supersymmetric particles, exist for each of the elementary particles. Hence, the existence of a heavier supersymmetric partner to the Higgs boson has already been predicted. If the new particle turns out to be a supersymmetric particle, it would qualify as a discovery.
2. the data reveals the existence of “extra dimensions” beyond the familiar three-dimensional space. The superstring theory, a prominent candidate as the ultimate explanation of all particles and forces, holds that all elementary particles including the graviton, which conveys gravitational force, are tiny, vibrating strings and that the universe is composed of 10 dimensions. The reason we are only aware of three dimensions, according to the theory, is that the remaining ones are small and curled up. Strings vibrating in this small space could be observed as particles with masses about the same as that seen in the recent round of accelerator experiments. If the existence of extra dimensions can be confirmed, that alone would be a major first.
3. the new particle is a “complex Higgs boson” made up of multiple elementary particles bound together. According to this line of thinking, the Higgs boson itself is not an elementary particle but rather consists of two unknown elementary particles, known as technifermions, stuck together. This would rewrite the Standard Model, built on 17 types of elementary particles, from the ground up.
We consider scenarios of warped extra-dimensions with all matter fields in the bulk and in which both the hierarchy and the flavor puzzles of the Standard Model are addressed. The simplest extra dimensional extension of the Standard Model Higgs sector, i.e a 5D bulk Higgs doublet, can be a natural and simple explanation to the 750 GeV excess of diphotons hinted at the LHC, with the resonance responsible for the signal being the lightest CP odd excitation coming from the Higgs sector. No new matter content is invoked, the only new ingredient being the presence of (positive) brane localized kinetic terms associated to the 5D bulk Higgs, which allow to reduce the mass of the lightest CP odd Higgs excitation to 750 GeV. Production and decay of this resonance can naturally fit the observed signal when the mass scale of the rest of extradimensional resonances is of order 1 TeV.
We discuss how to search for a possibe signal of the recently observed 750 GeV enhancement in the diphoton channel in the four-jet production. In the present studies we assume that the produced state is pseudoscalar. This fact, when combined with specificity of the corresponding amplitude, allows to improve the signal-to-background (S/B) ratio. We discuss in detail how to impose cuts on jets in rapidity and transverse momenta in order to find optimal S/B ratio and not to loose too much statistics. Our study suggest a measurement of two soft (low cut on pt) large-rapidity jets and two hard (high cut on pt) mid-rapidity jets. Azimuthal correlation between the soft external jets may be useful to further improve the situation. Several differential distributions in rapidities and transverse momenta of jets as well as dijet invariant mass are shown. The integrated cross sections corresponding to different cuts are collected in a table and number of events are presented.
In this paper we study the direct production of the diphoton resonance X which has been suggested by 2015 data at the LHC, in e+e−→Xγ/XZ processes at the ILC. We derive an analytic expression for the scattering amplitudes of these processes, and present a comprehensive analysis for determining the properties of X at the ILC. A realistic simulation study for e+e−→Xγ is performed based on the full detector simulation to demonstrate the capabilities of the ILC experiment. Complementary to the searches at the LHC, prospects of the measurement of the absolute values of production cross-section are obtained for the ILC using recoil technique without assuming decay modes of X. In addition, we have studied the searches for X→invisible and X→bb¯ modes, which are challenging at the LHC, and found that these decay modes can be discovered with high significance if their branching ratios are large enough.
If the gamma-gamma resonance at 750 GeV suggested by 2015 LHC data turns out to be a real effect, what are the implications for the physics case and upgrade path of the International Linear Collider? Whether or not the resonance is confirmed, this question provides an interesting case study testing the robustness of the ILC physics case. In this note, we address this question with two points:
(1) Almost all models proposed for the new 750 GeV particle require additional new particles with electroweak couplings. The key elements of the 500 GeV ILC physics program—precision measurements of the Higgs boson, the top quark, and 4-fermion interactions—will powerfully discriminate among these models. This information will be important in conjunction with new LHC data, or alone, if the new particles accompanying the 750 GeV resonance are beyond the mass reach of the LHC.
(2) Over a longer term, the energy upgrade of the ILC to 1 TeV already discussed in the ILC TDR will enable experiments in gamma-gamma and e+e- collisions to directly produce and study the 750 GeV particle from these unique initial states.
SOURCES – Arxiv, Nikkei