Uncovering a Higgs Hiding Behind Backgrounds

Hello particle munchers,

hidingHiggs
Figure 1: Monsieur Higgs boson hiding behind a background.

Last time I discussed the Higgs boson decay into photons, i.e. `shining light on the Higgs boson‘. This is a followup discussing more generally the problem of uncovering a Higgs boson which is hiding buried behind what can often be a large background (see Figure 1).

Perhaps the first question to ask is, what the heck is a background? Well, basically a background is anything that we `already know about’. In this case, this means the well understood Standard Model (SM) processes which do not involve a Higgs boson (which in this case is our `signal’), but can nevertheless mimic one of the possible decays of the Higgs. For most of these processes, we have very precise theoretical predictions in addition to previous experimental data from the LEP and Tevatron experiments (and others) which previously searched for the Higgs boson. So it is in reference to these non-Higgs SM processes when we use the term `background’.

As discussed in my previous post, the Higgs can decay to a variety of combinations of SM particles, which we call `channels’. Each of these channels has its own corresponding background which obscures the presence of a Higgs. For some channels the backgrounds are huge. For instance the background for a Higgs decaying to a pair of bottom quarks is so large (due to QCD) that, despite the fact this is the dominant decay channel (about 60% of Higgs’ decay to bottom quarks at 125 GeV), this channel has yet to be observed.

This is in contrast to the Higgs decay to four charged leptons (specifically electrons and muons) channel. This decay (mediated by a pair of virtual Z bosons) was one of the first discovery channels of the Higgs at the LHC despite the fact that roughly only one in every 10,000 Higgs bosons decays to four charge leptons. This is because this channel has a small background and is measured with very high precision. This high precision allows LHC experiments to scan over a range of energies in very small increments or `windows’. Since the background is very small, the probability of observing a background event in any given window is tiny. Thus, if an excess of events is seen in a particular window, this is an indication that there is a non background process occurring at that particular energy.

hto4lpeak
Figure 2: The energy spectrum of a Higgs decaying to four charged leptons (red) and its associated background (blue).

This is how the Higgs was discovered in the decay to four charged leptons at around 125 GeV. This can be seen in Figure 2 where in the window around the Higgs signal (shown in red) we see the background (shown in blue) is very small. Thus, once about a dozen events were observed at around 125 GeV, this was already enough evidence for experiments at the LHC to be able to claim discovery of the long sought after monsieur Higgs boson.

 Further Reading:

Seeking and Studying the Standard Model Higgs Particle

Decays of the Standard Model Higgs

The following two tabs change content below.
Roberto Vega-Morales is currently a post-doctoral researcher in high energy theory at the University of Granada in Spain. Previously he was at the Laboratoire de Physique Thèorique in Paris France. He conducted his Ph.D studies at Northwestern University as well as Fermilab and was awarded the 2014 J.J. and Noriko Sakurai Dissertation Award in Theoretical Particle Physics. His research focuses on the phenomenology of the Higgs boson at the LHC as well as models of Supersymmetry and extended Higgs sectors. He struggled mightily with French and is happy to be speaking Spanish nowadays.

Latest posts by Roberto Vega-Morales (see all)

Leave a Reply

Your email address will not be published. Required fields are marked *