Possible Evidence of New Particle in W + 2jets events ?
 Figure 1 : Illustration showing the decay mode from W+W/Z into a lepton pair and a quark pair. |
 Figure 2 : Feynman diagram corresponding to the above illustration. |
 Figure 3 : Invariant mass distribution reconstructed from two jets. A discrepancy between the data (black dots) and the expected background (height of red histogram) around 140 GeV/c². The blue line shows the distribution which best reproduces the data with an assumption that a new particle representing by a Gaussian (normal) distribution exists. |
 Figure 4 : Plot of the distribution after subtracting the expected background from the data shown in Figure 3. |
In a previous High Energy Physics Laboratory News, we reported on the observation of the simultaneous double production of W and Z particles, which mediate weak interactions, from the results of the CDF experiment in which our laboratory participates (for details, please refer to the High Energy Physics Laboratory News dated November 9, 2006, and February 1, 2008). At that time, measurements of the production cross-section, which indicates the likelihood of the production reaction occurring, were conducted. This time, in an analysis attempting to further improve the precision of the simultaneous double production cross-section measurement, we obtained data suggesting the potential existence of a new phenomenon that was not predicted at all by the Standard Model of particle physics, which is currently believed to be correct by many particle physicists. For measuring the simultaneous production cross-sections of WW and WZ, we use modes in which one W decays into a lepton (electron or muon) and a neutrino, while the other W or Z decays into a quark and an antiquark (Figures 1 and 2). The quarks and antiquarks resulting from the W or Z transform into jets of particles with high momentum in the same direction, and are detected.
The mass of an unstable particle can be calculated from the momentum and energy of its decay products, known as the invariant mass (invariant in this context means that it is the same for all observers, considering the theory of relativity). The anomalous behavior observed in the current data appeared in the invariant mass distribution obtained from the two jets produced along with the W that decayed into a lepton and a neutrino (Figure 3). What is expected is a peak near the mass of the W or Z that decayed into quark and antiquark, separate from the W that decayed into a lepton and a neutrino (red component in Figure 3). However, looking closely at Figure 3, there is a discrepancy between the expected distribution, obtained by summing all known peaks and backgrounds, and the data around 140 GeV/c2. To make it clearer, Figure 4 shows the data after subtracting the expected background.
There are uncertainties in the expected known backgrounds. Additionally, there is significant statistical uncertainty in the data. Therefore, it is possible that this distribution was obtained by chance in this experiment, even though there is no new particle. Based on our understanding of these uncertainties, we estimate the probability of obtaining the observed distribution without the existence of a new particle to be approximately once in 1375 experiments. This corresponds to 3.2 standard deviations in a Gaussian (normal) distribution. (A standard deviation is a measure of the amount of variation or dispersion in a set of values. A standard deviation of 1.0 means that 68 out of 100 results fall within this range, indicating a close average. As values deviate further from the mean, the standard deviation increases, and the probability decreases.) Once in 1375 may seem rare in everyday terms, but no one can predict when that one time will occur. In particle physics, we empirically require a deviation of 5 standard deviations (a deviation that would occur once in a million times due to background alone) to be confident that something different exists (a discovery), and this has become a convention. Therefore, we do not yet consider this anomalous distribution as definitive evidence of a new particle’s existence.
However, if it were true, what could it be? Including the manner of particle decay, the production cross-section would be over 1 pb (10−36 cm2), more than 100 times larger than the only undiscovered particle in the Standard Model, the Higgs particle, thus ruling out the possibility of this particle being the Higgs particle. If it is a new particle that cannot be explained by any of the currently proposed theories extending beyond the Standard Model, it would be a historically significant issue in particle physics, necessitating the formulation of a new theory incorporating this particle.
Currently, CDF continues to collect data, and we plan to carefully monitor whether this peak will grow or disappear in the future. Additionally, it is necessary to similarly investigate the 140 GeV/c2 region in experiments by the other experimental group at Fermilab, DZero, and at the Large Hadron Collider (LHC), a proton-proton collision accelerator constructed on the suburbs of Geneva, Switzerland.
- Article of News Site at Fermilab : Fermilab Today (April 7, 2011)
- arXiv preprint “Invariant Mass Distribution of Jet Pairs Produced in Association with a W boson in ppbar Collisions at sqrt(s) = 1.96 TeV”, arXiv:1104.0699[hep-ex]