In the last decades particle physicists have developed an enormously successful model for describing the known forces between particles, the 'Standard Model' . So far, the biggest success of the Standard Model is the unification of the electromagnetic and the weak force into the so-called electroweak force. Each force is carried by characteristic particles called bosons . The photon carries the electromagnetic force; it also transmits light. The W and Z bosons represent the weak force.
The electroweak unification leads to a problem: it requires all particles which carry forces to be massless like the photon. But experiments show that the W and Z bosons are not massless at all; their mass is comparable to the mass of a gold nucleus.
Introducing a new field, the Higgs field, solves this problem. It interacts with W and Z bosons to give them mass. The Higgs field gives rise to a new particle, called the Higgs boson, which has not yet been observed. It is the keystone of the Standard Model and finding the Higgs boson is one of the primary goals of the DØ Experiment.
The Standard Model predicts exactly one Higgs boson which carries no electric charge. But nature might have chosen a more complicated path, leading to several Higgs bosons with different properties.
We already know now that the Standard Model is not the final answer to all our questions since there are phenomena in particle physics for which the Standard Model makes predictions which contradict basic physical principles. In addition, it just describes many phenomena but does not explain why these phenomena occur. Many of the new theories which try to answer these questions predict the existence of several different Higgs bosons.
One set of models predicts a 'triplet' of Higgs bosons: one neutral Higgs boson, one charged Higgs boson which has the same electric charge as an electron, and one 'doubly-charged' Higgs boson which has twice the electric charge of an electron. In the paper presented here we are trying to find the doubly-charged Higgs bosons (H++ or H--).
The DØ Experiment is designed to measure particles produced in the collision of protons and antiprotons at very high energy. At these high energies, protons and antiprotons break up and we can observe the interactions of their building blocks, quarks and gluons. If doubly-charged Higgs bosons exist, they can be produced in pairs by the annihilation of quarks and antiquarks. This is the process we are looking for:
The doubly-charged Higgs bosons are not directly observed in the detector. They will decay into other particles almost immediately after they have been produced. Only these particles are stable, they traverse the DØ Experiment and leave traces in the detectors. We do not know into which particles the doubly-charged Higgs bosons will decay. In this search we assume that they will decay only into muons . Decays into other particles, such as electrons or tau-leptons are also possible and we will study them in the future.
For now, we just look for muons. Muons are very similar to electrons, only about 200 times heavier. One doubly-charged Higgs boson would decay into two muons of the same electric charge. This violates a rule of the Standard Model which is related to counting the number of muons and muon-neutrinos in a process, since the doubly-charged Higgs bosons are part of new theories which predict violations of this rule.
Each recorded proton-antiproton collisions is called an 'event'. In our doubly-charged Higgs events, we expect four muons, two positively charged and two negatively charged. Muons have two properties that make it relatively easy to detect them in the DØ Experiment. They are electrically charged and they can traverse thick layers of material without being absorbed.
Many electrically charged particles are produced in proton-antiproton collisions. They all leave tracks in the detectors surrounding the vacuum pipes where protons and antiprotons collide. Outside this first layer of detectors there are several layers of material consisting of other detectors (calorimeters) and of magnets. Most particles are absorbed in this material and they can therefore not reach the outermost layer of detection, the so-called muon-chambers. The rest is easy (in principle): If a particle left a signal in the outermost layers (muon chambers) and in the inner layers of the detector, we know it is a muon.
Two doubly-charged Higgs bosons would produce four muons which should leave a signal in our detector. However, our detector is not completely hermetic,and we are 'blind' in those regions where the vacuum pipe for the proton and antiproton beams traverses it. In some cases we therefore expect to observe only two or three muons from our signal.
After sifting through all the data, three candidate events remain with two or more muons in them. These events could originate from the doubly-charged Higgs bosons we search for. One of the candidate events with three muons is shown in this computer reconstruction :
We look at the detector from the proton beam direction. The scale of the picture is about 10 metres by 10 metres.
The final question is:
This is not quite the end of the story. Using all the information extracted from our data, in particular, the number of candidate events expected if a doubly-charged Higgs boson existed and the actual number of candidate events observed, we can learn more: If the specific doubly-charged Higgs boson we are searching for exists, it must have a mass greater than about 118 GeV (about 126 proton masses). Even though this is not a discovery yet, this limit is very important for constructing and testing new theories . We currently give the world's best published limit for the hypothetical doubly-charged Higgs boson, about 20% better than the previous results from the electron-positron collider LEP at CERN .
Fifty times more proton-antiproton collision will be recorded in the next years by the DØ Experiment. This is therefore just the beginning in our quest to find the elusive Higgs bosons.
If you have any questions about this research, please contact Stefan Söldner-Rembold (University of Manchester, UK), Marian Zdrazil (State University of New York at Stony Brook, USA) or Avto Kharchilava (University of Notre Dame, USA).