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19/11/2007 SOCLE@Clermont 1
Manqi RuanDiscussing & Support: Francois, Roman, Vincent, Advisor: Z. ZHANG (LAL) & Y. GAO (Tsinghua))
Higgs mass measurement through μ channel of Higgs strahlungs process
(e+e-HZμμH)
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Outline• Motivation & Software introduction
• Preselection cuts at generator level
• Higgs Mass & Xsection determination– Result for SM Higgs– Result if Higgs can decay invisibly
• Summary
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Motivation: • Higgs strahlungs process:
• Higgs Recoil Mass
• X section measurement: coupling strength
sEmsm ZZh 222
LNg /2
Higgs invisible decay
Higgs SM decay
μ +
μ -
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Sqrt(s) = 230GeV:
Recall the analytic form of error on Higgs mass:
δmh2 = sqrt{((4Ep1-mZ
2)p1k(p1))2 + ((4Ep2-mZ2)p2k(p2))2 } ~ p2 ;
Small sqrt(s) means better Higgs mass resolution!
Motivation: Why Higgs strahlung:
Only muon momentum information is needed inHiggs strahlung channel analysis
A model independent (without any assumption on Higgs decay) analysis can be applied
Any potentially model dependentcut will not be used here
Beam polarization will increase the signal cross section by 58%. (electron 80%, positron, 40%)ISR effect will reduce the cross section with sqrt(s)<300GeV while increase it a little at higher energy
Xse
c/fb
Hmass = 120 GeV
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Software chain• Generator: whizard-1.50, pythia 6.4.13 ( with Guinea-Pig
to simulate BS effect: Thanks to cecile! );
• Full Simulation: Mokka-v06-04. with LDC01_sc detector conception (184 TPC layer), the accuracy of tracking system to 5E-5 at δ(1/P) on average
• Reconstruction & Analysis: MarlinReco/Marlin, ROOT;
• Fit: RooFit package
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(Invisible) ZH(fb) 6.62 (3310 evt) 7.78 (3890) 4.87 (2435)
eeZZμμvv (fb) 15.42 (7710) 14.61 (7305) 10.18 (5090)
eeWWμμvv (fb) 207.96 (104k) 198.42 (99.2k) 146.44 (73.2k)
Sqrt(s) 230GeV 250GeV 350GeV
(SM) ZH(fb) 6.62 (3310 evt) 7.78 (3890) 4.87 (2435)
eeZZ (fb) 1.34k (672k) 1.27k (635k) 0.856k (428k)
eeWW (fb) 15.86k (7.93M) 15.61k (7.81M) 1.155k (5.77M)
eeqq (fb) 57.6k (28.8M) 52.2k (26.1M) 22.63k (11.3M)
X section of main BG
Non-Polarized beam at 500 fb-1; ISR, BS actived
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Pre cuts introduction
• If Higgs invisible decay, no pre cut
• If N_track > 3, (SM Higgs decay), apply following pre cut (pions are included here for the PID have a chance ~1% to misidentify the a pion as a muon)– Energetic pion/muon (E1>15GeV )
– Exist another pion/muon (with energy E2), together with the most energetic pion/muon to form a invariant mass > 70 GeV
– Dynamic cut: 2E1+E2<180&&2E1+3E2>200
– cos(θmumu)>-0.95
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Higgs SM decay
Huge SM background:(qq, WW, ZZ)Pre cut is needed beforeProcess to Full Simulation!
Current muon id efficiency ~ 93.6% purity ~ 99%(O.Martin, RDR)
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• 3 Pre cuts to reduce the qq back ground– Energetic muons > 15GeV– Invariant mass of Muons >
70 GeV– cos(θmumu)> -0.95
• A few hundreds qq Events survive, far from signal region (115 -140GeV): qq back ground vanishes after pre cut selection
qq background
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ev μv τv qq
ev
μv
τv
ee μμ ττ vv qq
evμμττ
νν
ZZ & WW background
Blue: background for Higgs invisible decayGray: background for Higgs invisible decay through tau leptonic decayLight green: background for Higgs SM decayRed, pink and light blue: possible background for Higgs SM decay (pion be misidentified as muon & muon from bb, cc)Yellow and Dark Green: background for Higgs SM decay: H ττ
ZZ WW
Z decay ratio: ~3% to lepton pairs (each), ~20% to neutrino pairs, ~70% to qq
W decay ratio: ~10% to lepton pairs (each), ~70% to qq
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Pre cuts effect on WW & ZZ background
• Effect: 56k (of 7.9M) WW events & 11.3k (of 672k) ZZ events survive– Of the 56k WW events;
• 220 events with 2 muon• 46.7k events with muon + pion • 10.55k events with 2 pions• With current PID ~ 400 WW events remains after require both
muons identified (reconstruction efficiency ~ 64%)
– Of the 11.3k ZZ events;• 10.26k μμqq events (majority)• 410 ττqq events • 162 qqqq events• 1.06k 4-lepton events• 8.6k ZZ events remains after reconstruction with require both
muons identified (reconstruction efficiency ~ 79%)
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S+B for Higgs SM decay. Br(SM)=100%
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ZH ZZ WWTotal EventNum at 500 fb-1 3310 11k 630
Both muon identified 2714 (82.0%) 8638 (78.5%) 400 (63.5%)
|cos(θ)|<0.99 2710 8621 400
Emu>20 2693 8531 318
2E1+E2<182&&2E1+3E2>202 2672 7218 289
cos(θmumu)>-0.95 2462 7022 259
|mZ-mlepton|<10 2363 (71.4%) 5739 (5.74%) 80 (12.7%)
Cuts Chain in Higgs SM decay
Require both muon be identified will reduce our efficiency by ~17%, but also reduce greatly the back ground a muon is more easily to be misidentified in forward region Cut applied on the reconstructed data is more strict than the pre cut at MC truth level
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S+B after cuts & Gaussian like background
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Fit 2 parameters with likelihood method : mH & Fraction
mH = 119.999 GeV ± 18.0 MeV
Frac = 54.93%±1.4% (δσ ~ 0.19 fb)
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Higgs Invisible decay
Exotic Model beyond SM:
SUSY ?Extra dimension ?Heavy neutrino ?
…Main background
e+e- WW, ZZ μμvv
Higgs Decay Br with a forth neutrino
mHwith mass = 50 GeV
K.Belotsky hep-ph/0210153 (2002)
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S+B for Higgs invisible decay. Br(inv)=100%
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Cuts Chain in Higgs invisible decayZH ZZ WW
X Section (fb) 6.62 15.42 207.96
Total EventNum at 500 fb-1 3310 7710 103980
Reconstructed event num & efficiency (with mH resolved)
3260 (98.5%) 7614 (98.8%) 96661 (93.0%)
|cos(θ)|<0.99 3230 (97.6%) 7566 (98.1%) 96157 (92.5%)
|mZ-mlepton|<10 3091 (93.4%) 7134 (92.5%) 11570 (11.1%)
Emu>20 3091 (93.4%) 7129 (92.5%) 11570 (11.1%)
cos(θmumu)<-0.4 3091 (93.4%) 4765 (61.8%) 6868 (6.61%)
Total energy<110 3086 (93.2%) 1762 (22.9%) 4827 (4.64%)
Cut on W mass resolve:
(2<Ratio<4)
2874 (86.8%) 1165 (15.1%) 3278 (3.15%)
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S+B after cuts & Gaussian like background
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Fit 2 parameters with likelihood method : mH & Fraction
δmH = 16.5 MeV
δ(Frac) = 1.2% (δσ ~ 0.18 fb)
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For arbitrary Br(inv)Combination the result fromHiggs invisible and visible decay( Br(inv) + Br(visible) =100% )Red, invisible part contributionBlue, visible part contributionGreen, overall result
δmH < 18 MeV
δσ < 0.23 fb
σ ~ 6.6 fb
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Summary• Accuracy of Higgs mass and cross section measurement through
eeHZHμμ with Higgs SM decay and Higgs invisible decay assumption have been studied.
• An accuracy of about 18MeV on Higgs mass measurement can be achieved at 500 fb-1 with non polarized beam. ( Details: δ(mH) = 13.6 MeV for Pure signal, δ(mH) = 16.5 MeV with Br(inv) = 100%, δ(mH) = 18 MeV with Br(SM) = 100% ). Overall reconstruction efficiency is 71.4% for SM Higgs decay case, and 86.8% for Higgs invisible decay
• Cross section can be measured to an accuracy of 3% level
• It is foreseen to improve a lot with beam polarization for it will not only reduce the WW background but also increase ~58% the cross section of Higgs strahlung channel (electron, 80%, positron, 40%).
• To do: Generate samples with enough statistic to optimize the fit. Optimize the cuts to save signal efficiency. (Especially for Higgs SM decay, with using the jet information. )
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Back Up Slides
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Fit Algorithm• Generate 2 samples with mH=119GeV and mH=121 GeV
assumption (currently 10k statistic each).
• Getting PDF f(119,m), f(121,m): smooth the Higgs recoil mass spectrum to a PDF function.
• Assume the higgs mass is ma ( 119< ma <121); then the expected PDF f(ma,m) will be a linear combination of f(119,m-(ma-119)) and f(121,m+(121-ma)): (Shift the PDF(119) & PDF(121) to expected position and make a linear combination)
f(ma) = 0.5*((ma -119)* f(121,m+(121-ma))+ (121-ma)* f(119,m-(ma-119)) )
• Use likelihood method to fit ma
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How the likelihood method works:
Smoothed mH Spectrum with mH=121/119 assumption:Then shift to measured mH andtogether form an expected PDF at mH=120 (Green)Match the sample data nicely
sample with 10k statistic each
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Red/Green/Blue: Smoothed mH Spectrum with mH=121/120/119 assumption and original Histogram (show in error bar);Yellow: expected PDF at mH=120, calculated from f(119) and f(121) match to the sample (Green) nicely. 10k statistic for mH=121 & mH=119 cases; 6k statistic for mH=120 case.
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mH measurementwith no background
IO test at 500fb-1
Use toy MC provide by RooFit:
Num of events per run is determinedBy a gaussian distributionGaus(N, sqrt(N)). Totally 500 run
Error on mH nicely agreed withError on sample means distribution:
IO test fine
δmH = 13.7 MeV
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mH measured: 119.966GeV±16.6MeVTotal Event Num with 115<mH<140:2871 events + 3202 background;Signal Fraction = 2871/(2871+3202) = 47.275%Measured Fraction = 47.0% ± 1.2%
Fit 2 parameters with likelihood method : mH & Fraction
There is a displacement of about 2 sigma (34 MeV,) which could be eliminate using much meticulous samples (large statistic + detailed Higgs mass assumption)
δmH = 16.5 MeV
δ(Frac) = 1.2% (δσ ~ 0.18 fb)
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IO test with toy MC at 500fb-1. Br(invisible) = 100%. 1000 Samples
δmH = 16.5 MeV
δ(Frac) = 1.2%
Truth: 47.275%.Measured: 47.28%±1.2%
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X Section measurement Br(inv)=100%
• TotalEventNr*fraction=BranchingRatio*efficiency*luminosity*Xsection• Efficiency = 86.83%; luminosity = 500 fb-1
• Result: 6.613±0.181 fb. Accuracy at 2.7% level
MCTruth:σ: 6.62 fbBr: 100% (invisible decay)
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Dynamical Pre CutFor SM Higgs decay
2*E1+E2<180 && 2*E1+3*E2>200
Effect: Signal Channel: 98.8%Back Ground WW: 25.4% ZZ: 31.9%
Signal
WW
ZZ
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Effect of individual Cuts for Higgs invisible decay
ZH ZZ WWX Section (fb) 6.62 15.42 207.96
Total EventNum at 500 fb-1 3310 7710 103980
Reconstructed event num & efficiency (with mH resolved)
3260 (98.5%) 7614 (98.8%) 96661 (93.0%)
|cos(θ)|<0.99 3229 (97.6%) 7566 (98.1%) 96157 (92.5%)
|mZ-mlepton|<10 3120 (94.3%) 7175 (93.1%) 11626 (11.2%)
Emu>20 3248 (98.1%) 7584 (98.4%) 94128 (90.5%)
cos(θmumu)<-0.4 3211 (97.0%) 5123 (66.4%) 74202 (71.4%)
Total energy<110 3253 (98.3%) 2056 (26.7%) 25107 (24.1%)
Cut on W mass resolve:
(2<Ratio<4)
2956 (89.3%) 1407 (18.2%) 32444 (31.2%)
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Cut on W mass resolve: (2<Ratio<4)
2<Ratio<4
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Momentum resolution vary with energy: 10o, 15o, 20o, 30o ,40o, 60o, 80o polar angle: yellow curve is the Fast simulation result from M.Berggren
• Thanks to hengne’s work on the SIT radiation length, we have gain a factor of 5% improvement in the mu momentum resolution
5*10-5