HIG-11-033

"Search for the standard model Higgs boson decaying into two photons in pp collisions at √s=7 TeV"

This is a condensed description with plots for the analysis CMS-HIG-11-033. (Phys. Lett. B 710 (2012) 403-425)

Table of contents

Abstract

A search for a Higgs boson decaying into two photons is described. The analysis is performed using a dataset recorded by the CMS experiment at the LHC from pp collisions at a centre-of-mass energy of 7 TeV, which corresponds to an integrated luminosity of 4.8 inverse femtobarns. Limits are set on the cross section of the standard model Higgs boson decaying to two photons. The expected exclusion limit at 95% confidence level is between 1.4 and 2.4 times the standard model cross section in the mass range between 110 and 150 GeV. The analysis of the data excludes, at 95% confidence level, the standard model Higgs boson decaying into two photons in the mass range 128 to 132 GeV. The largest excess of events above the expected standard model background is observed for a Higgs boson mass hypothesis of 124 GeV with a local significance of 3.1 sigma. The global significance of observing an excess with a local significance greater than 3.1 sigma anywhere in the search range 110-150 GeV is estimated to be 1.8 sigma. More data are required to ascertain the origin of this excess.

Further details

To improve the sensitivity of the search, selected diphoton events are subdivided into classes according to indicators of mass resolution and signal-to-background ratio. Five mutually exclusive event classes are defined: four in terms of the pseudorapidity and the shower shapes of the photons, and a fifth class into which are put all events containing a pair of jets passing selection requirements which are designed to select Higgs bosons produced by the vector boson fusion process.

The background model is obtained by fitting the observed diphoton mass distributions in each of the five event classes.

The local p-value quantifies the probability for the background to produce a fluctuation at least as large as observed, and assumes that the relative signal strength between the event classes follows the MC signal model for the standard model Higgs boson. The local p-value corresponding to the largest upwards fluctuation of the observed limit, at 124 GeV, has been computed to be 9.2x10^-4 (3.1 sigma) in the asymptotic approximation, and 1.5x10^-3(3.0 sigma) when the calculation uses pseudo-data. The combined best fit signal strength, for a SM Higgs boson mass hypothesis of 124 GeV, is 2.1+-0.6 times the SM Higgs boson cross section. Since a fluctuation of the background could occur at any point in the mass range there is a look-elsewhere effect. When this is taken into account the probability, under the background only hypothesis, of observing a similar or larger excess in the full analysis mass range (110 < mH < 150 GeV) is 3.9x10^-2, corresponding to a global significance of 1.8 sigma.

Figures from the paper

Image	File links	Description
	pdf png a) pdf b) pdf c) pdf d) pdf e) pdf f) pdf	Figure 1: Background model fit to the mgg distribution for the five event classes, together with a simulated signal (mH=120 GeV). The magnitude of the simulated signal is what would be expected if its cross section were twice the SM expectation. The sum of the event classes together with the sum of the five fits is also shown. The individual graphs (available as separate files) are as follows: a) The sum of the five event classes, b) the dijet-tagged class, c) both photons in the barrel, R9min > 0.94, d) both photons in the barrel, R9min < 0.94, e) at least one photon in the endcaps, R9min > 0.94, f) at least one photon in the endcaps, R9min < 0.94.
	pdf png	Figure 2a: Exclusion limit on the cross section of a SM Higgs boson decaying into two photons as a function of the boson mass.
	pdf png	Figure 2b: Exclusion limit relative to the SM Higgs boson cross section, where the theoretical uncertainties on the cross section have been included in the limit setting. On popular demand: the observed and expected limit at mH = 125 GeV are 3.14 and 1.43, respectively.
	pdf png	Figure 3: The local p-value as a function of Higgs boson mass, calculated in the asymptotic approximation. The point at 124 GeV shows the value obtained with a pseudo-data ensemble.
	pdf png	Figure 4: The best fit signal strength, in terms of the standard model Higgs boson cross section, for the combined fit to the five classes (vertical line) and for the individual contributing classes (points) for the hypothesis of a SM Higgs boson mass of 124 GeV. The band corresponds to +-1 sigma uncertainties on the overall value. The horizontal bars indicate +-1 sigma uncertainties on the values for individual classes.

Vertex finding, pileup and data/MC comparison

Image	File links	Description
	pdf png	Fraction of Higgs boson vertices found within 10 mm of their true location, for a Monte Carlo signal sample (mH = 120 GeV), as a function of the Higgs boson transverse momentum. The distribution of the number of interactions per bunch crossing (nPU) in the Monte Carlo is adjusted to be the same as in the data by weighting the events.
	pdf png	Distribution of the number of reconstructed vertices (using the deterministic annealing algorithm) for events with a Z decaying to 2 muons in the data (points) and MC after having applied the reweighting on the number of simulated in-time pile-up events (filled histogram). The distributions are shown for 2 running periods: 2011A - up until late August, during which time the average instantaneous luminosity and consequent pileup was lower, and the time after that, 2001B, when the instantaneous luminosity was higher.
	pdf png
	pdf png	Diphoton mass distribution for data (data points), and Monte Carlo simulation of SM processes which constitute the background to the search (histograms). A simulated signal for a Higgs boson with a mass of 120 GeV is shown by the red outline histogram. The magnitude of the signal is what would be expected if its cross section were 10 times the SM expectation.The shaded band represents the theoretical (k-factor) uncertainty on the the MC prediction. Note that in the analysis the background is evaluated from a fit to the data without reference to the MC simulation.

Energy resolution

Image	File links	Description
	pdf png	Invariant mass distribution of electron pairs from Z boson decay: where (top) both electrons are in the central barrel and have R9>0.94, and (below) where both electrons have R9>0.94, but one electron is in the central barrel and and the other is in the endcap. The red histogram is Monte Carlo, and the black empty histogram is the same Monte Carlo but after smearing of the energies to match the data.
	pdf png

Efficiency and acceptance

Image	File links	Description
	pdf png	Photon selection efficiency for mH=120 GeV as a function of as a function of pT, for each of the four photon categories.
	pdf png	Photon selection efficiency for mH=120 GeV as a function of as a function of pseudorapidity, for each of the four photon categories.
	pdf png	Efficiency x Acceptance as a function of mass for selected photons (all event classes combined). The band indicates 1 sigma systematic error.

Event display

png

3D display of event 625786854 in run 177201. A dijet-tagged event with diphoton mass 121.9 GeV, dijet mass 1460 GeV, photon transverse momenta 193.9 and 78.0 GeV, and jet transverse energies 288.8 and 189.1 GeV. The photon pseudorapidities are -0.405 and 0.037, and the jet pseudorapidities are -2.022 and 1.860.

Tables

Photon identification efficiencies measured in the four photon categories using a tag and probe technique applied to Z to ee events (for all requirements except the electron veto). Both statistical and systematic errors are given for the data measurement (in that order), and these are combined quadratically to calculate error on the ratio eff(data)/eff(MC). Available as png and pdf.

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Number of selected events in different event classes, for a SM Higgs boson signal (m_H = 120 GeV), and for data at 120 GeV. The value given for data, expressed as events/!GeV, is obtained by dividing the number of events in a bin of +-10 GeV, centred at 120 GeV, by 20 GeV. The mass resolution for a SM Higgs boson signal in each event class, is also given. Available as png and pdf.

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Separate sources of systematic uncertainties accounted for in this analysis. The magnitude of the variation of the source that has been applied to the signal model is shown in the second column. Available as png and pdf.

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Systematic uncertainties for the photon energy scale, for separate regions of detector and for photons with R9>0.94 and R9<0.94. Available as png and pdf.