Neutrino effect on dijet mass

I still did not cross check the effect of neutrinos in the dijet mass yet. In confirming so, we can take advantage of that to separate the ZH events from the ZZ background. The ZH events for dijet mass values below about 110 GeV should have more missing energy than the ZZ events with dijet mass above about 92 GeV. This range, 92 < dijet mass < 110, contains most of the overlapping between the two processes.  Then instead of cutting on the dijet mass at 96 GeV we cut on the missing ET-dijet mass plane. By just applying the cut, obtained "by eye", shown in the plot 10% of the signal was recovered whereas 10% more background events were cut when compared with the case where just a dijet mass cut is applied (see previous post).

If time allows I will think of a smart way to perform the background separation using the information from the plot below. But the idea is there.

Neutrino effect on the di-jet mass

Cheated assignment with and without Monte Carlo neutrinos
This plot shows the reconstructed di-jet mass, but using the reconstructed particles chosen from Monte Carlo. There's also the same thing but with the Monte Carlo neutrinos taken into account, which shows the low Higgs tails are clearly due to neutrinos.

Minutes of Meeting 14/8/08

Present: Roberval, Clare, Mark, Joel

• Roberval showed plots of his studies on the 230 GeV sample of Z->µµ+H, some of which have been discussed before. There was discussion of a dijet mass cut vs the recoil mass cut, but the dijet mass from the Higgs shows a long tail at lower values. Using a dijet mass plus Z energy cut does give very good background rejection for a signal efficiency of about 1/3.



• Mark's plots from Monday for the 250 GeV Z->ee samples are similar, but have been showing a high tail in the dijet mass spectrum (particularly in the ZZ).



• After some further investigation and discussion in the meeting, it was decided that:
  • The long low-mass tail in the Higgs dijet mass plots is probably neutrinos. Mark and Roberval should check this by adding
    in neutrinos from the MC
  
  
  
  • The high-mass tail in the electron samples is mainly due to photons (primarily FSR and Brehmsstrahlung) being included in the jets. Mark is looking at using a smaller Ycut to isolate photons (and taus) and then veto any jet with less than two tracks, as shown in his latest plots. He has also looked at picking out photons close to primary electrons.
    



• There was some confusion about whether we are expected to give a status report in the ILD meeting next week [it's actually the week after]. Since Victoria gave the last talk, it was decided that someone from Bristol (Mark) should give the next one.

Questions about beamstrahlung and ISR

I've been reading about initial state radiation (ISR) and beamstrahlung and I am not sure whether one can measure the photon or not. Some say that it is possible for ISR, but I understand that in most of the events the photon(s) would be outside the detector acceptance. Besides that the calculations of ISR that I know are done considering the electrons as having some sort of structure.

For beamstrahlung it is said it could be possible to measure its effects, using for example the lumi detector, considering the effect as a collective effect of the particles of the beam. But the nature of the effect is statistical. What if the particle participating in the hard scattering were affected? Moreover, no photon should be possible to be measured because the radiation is emitted as the beam moves along the tunnel.

Does anybody understand these issues better? I believe that understanding this things better we could try to do something to improve the measurements in a long term.

Plots for meeting

Yet again I've left it a bit late to describe these here, I'll talk about them in the meeting and then write something here later.

Mass plot for ycut jet finding
We had a go at selecting the hadronic stuff by using ycut jet finding with a low ycut and requiring the jets to have at least 2 tracks. This is the mass plot for the best peak I get.


Comparison
This is that plot overlaid on the perfect particle assignment from Monte Carlo, and the plot from removing photons close to the electrons. The ycut method has a slightly smaller peak than the brem removal method, but smaller high tail which is the important thing.

Status of the analysis

A presentation with the status of what I am doing for the analysis can be found here.

Dijet mass tail: Pandora x Pythia

I got a sample generated with pythia from the grid to compare with my Pandora-Pythia samples. Both shows long tails towards negative dijet mass values. I used 2000 events from each sample.

Improving the mass plots

When checking the performance of the precuts we found that the jet mass cut was cutting very little of the ZZ background out; there should be similar numbers of ZH and ZZ going into the likelihood cut but we had about 4 times as much ZZ. The mass plot (here, for cheated electron identification) is quite wide and has a long high tail on the ZZ.
We had a look at several things, like trying ycut jet finding in case it was initial state radiation photons being lumped in as well when forcing to 2 jets. Nothing conclusive came up so I looked directly at a few events with the di-jet mass over 200 GeV. They all seemed to have highly energetic photons from the Z (the one that goes to electrons) or from the electrons themselves lumped into the jets. I had a go at cutting out any photons (as identified by Pandora) that are within 4 degrees of the primary electrons and came up with this. Here's a plot of the energy and number of these photons.
After checking that I thought I'd try and find out if there was anything else being thrown in that shouldn't. From the Monte Carlo to reconstructed particle relations I listed all the reconstructed particles that come from each Z (for ZZ; or Z and H for ZH) separately and plotted the invariant mass for these. That's basically the best you could ever get without improving the particle flow/tracking/etcetera. Here's a plot of the 3 methods over laid for the ZZ sample - original cheated electron ID; bremsstrahlung removed and completely cheated jet association. Removing the bremsstrahlung photons does a pretty good job, but there's still something going into the jets that shouldn't. Here's some details about the particles that are left over when assigning to the Z or the H (or the Zs for the ZZ sample) from Monte Carlo.

Next steps:

• Find out what these left over particles are.
• See how the mass plots look using realistic electron finding and removing the bremsstrahlung.
• See how many ZZ and ZH get through the precuts now.

Minutes of Meeting 28/7/08

Present: Hajrah, Robervaal, Victoria, Clare, Mark, Joel (i.e. everyone)

• Clare and Mark have been having another look at electron ID. When Mark looked a few months ago, the default efficiencies looked rather poor due to close brehmsstrahlungs and separated parts of the EM shower so he has been using MC information to cheat since. Now things look much better: using either Pandora defaults or a Kuhl-style simple ID (one cluster matched to one track, with cuts on track isolation and EM vs Hadronic energy) efficiencies  above 90% are reached.  Further tuning will be done by Bristol MSci students.
• Robervaal has been looking at samples of ZH with Z->µµ and H->anything  at √s = 230 and 250 GeV (20,000 events in each). He has identified cutting on the Z recoil mass as an extremely promising way of reducing the ZZ background. 
• It seems clear that the best way to perform both electron and muon analyses is to identify the leptons and then remove them before jet clustering.
• Victoria has been looking at the B and C-tagging performance in jets. The C-tag does not look very good and probably needs re-tuning. The B/C tag is not relevant for this analysis.
• Hajrah checked the electron ID cuts in the new detector model. The EM/total and E/p cuts need to be changed. This was done in single particle simulations, so she will next check in physics event.
• Mark had some technical problems getting all of his plots on the web and will put them up after the meeting. The basic summary was that the flavour tag likeliness plots look good and we are probably ready to fit the templates, the last step in the analysis chain.

It was decided that we have enough to show in Wednesday's ILD meeting, and that Victoria will put together a talk.

ycut versus njet

I had a quick look into the ycut versus njet. You can see plots and conclusions here. It seems fine to use the njet mode instead of ycut to reconstruct the higgs from jets, if necessary.

What I am not sure is whether it is fine to use the 2-jet mode for vertexing and flavour tagging. At least the ZVKIN algorithm uses the jet axis as a starting direction for the ghost track. How the algorithm behaves when the starting direction is far away needs to be investigated.