Detection of Z Gauge Bosons in the Di-muon Decay Mode in CMS

Transcription

1 Detection of Z Gauge Bosons in the Di-muon Decay Mode in CM Robert Cousins, Jason Mumford and Viatcheslav Valuev University of California, Los Angeles for the CM collaboration Physics at LHC Vienna, -7 July 4 Physics at LHC, -7 July 4

2 Z : Introduction Generic name for additional heavy neutral gauge bosons. Appear in many models beyond the M: GUTs, dynamical symmetry breaking, little Higgs, etc. Might be light enough to be accessible at the LHC. Experimentally, Z µ + µ channel is one of most promising. Main Z µ + µ physics goals: Understand discovery potential and optimize the search strategy main observable: M µ+µ. This talk Work out algorithms to distinguish among models (including graviton) and study properties, once discovered Work in progress main observables: A FB on- and off-peak, y, σ Br, Γ. Physics at LHC, -7 July 4

3 Z µ + µ studies in CM Rich history of studies of Z discovery potential in CM: Pioneered by C.-E. Wulz in Later studied by D. Bourilkov (), JIR group (. hmatov et al., -) and ETH group (M. Dittmar et al., ). Motivations to do more work now: More realistic estimates can be obtained thanks to new tools available: Move from event-generator-level studies to full simulation, reconstruction, and trigger emulation; study systematic uncertainties. Use well-defined statistical techniques to quantify the mass reach. Z µ + µ is a good benchmark channel for the muon detector, largely complementary to other channels (like H 4µ, H µν, etc.): Reconstruction of very-high-p T muons. tudy of detector misalignment, B field uncertainties, calibration uncertainties, etc. Chosen to be studied extensively for CM Physics TDR (late 5) Physics at LHC, -7 July 4

4 Z µ + µ : signal and backgrounds Z s arise in many models; we studied six frequently discussed: Z M in sequential standard model (benchmark toy model); Z = Z ψ cosθ E6 + Z χ sinθ E6 in E 6 and/or O() models: Z ψ, Z χ and Z η (θ E6 = 7.78 o ); Z LRM and Z ALRM in left-right symmetric models. Limits on the mass: Current: 6-8 GeV; Expected by LHC start-up: TeV. Br(Z µ + µ ): -8% (if no exotic decay channels are open) PYTHIA cross-sections; no added K-factors dσ/dm (fb/4 GeV) BR tt DY di-bosons TeV Z M M µµ (GeV) Dominant (and irreducible) background: Drell-Yan production of muon pairs pp γ/z µ + µ. Other sources: ZZ, WZ, WW, tt, bb, pile-up, etc. (much smaller than Drell-Yan and reducible) Physics at LHC, -7 July 4 4

5 Generation and reconstruction Event generation: PYTHIA6, CTEQ5 PDF s Z M, Z ψ, Z η, Z χ, Z LRM, and Z ALRM at, and 5 TeV decaying to µ + µ Obtain couplings from literature, introduce in PYTHIA. Include the full γ*/z /Z interference structure. Drell-Yan µ + µ pairs in several mass intervals. imulation and reconstruction: Detector response: full GEAT-based description (CMIM). Event reconstruction: CM OO reconstruction package (ORCA). Includes: digitization (detailed simulation of electronic response); emulation of Level- and High-Level (HLT) Triggers; off-line reconstruction (our group worked on algorithms capable of improving momentum resolution for very-high-p T muons). Physics at LHC, -7 July 4 5

6 Event selection Both µ + and µ should be within the geometrical acceptance of the muon system < η ).4) and pass the trigger: Acceptance: raises from about 8% at TeV to almost 95% at very high M µµ. Acceptance M + - µ µ (GeV) L/HLT trigger efficiency: about 98% at TeV, about 95% at 5 TeV. Require that there are at least two µ s of opposite charge sign. o background-rejection cuts. Overall efficiency: 7-75% (-5 TeV). Physics at LHC, -7 July 4 6

7 Fitting procedure Generate ensembles of MC experiments: number of events in each experiment fluctuates according to Poisson distribution with a mean of σ Br ( Ldt) ε; appropriately add Drell-Yan contribution from lower masses. In each experiment, fit M µµ values using an unbinned maximum likelihood: p( M µµ ) = s s ps ( M µµ ; m, Γ ) + pb ( M µµ ) tot tot p s (signal pdf) is a convolution of a Breit-Wigner with a Gaussian smearing; p b (background pdf) is an exponential, with the slope parameter determined from fits to Drell-Yan events. Up to three free parameters: signal fraction ( s / tot ), signal mean (m ), and signal FWHM (Γ). o constraints on the absolute background level: fit assumes only background shape is known. Physics at LHC, -7 July 4 7

8 Example: Z ψ at TeV, Ldt =. fb - - Events/5 GeV/. fb - Events/ GeV/. fb mass (GeV) + - µ µ Z DY L = 7.7 Entries 8 Mean 78.9 RM 6. - Events/5 GeV/. fb - Events/ GeV/. fb mass (GeV) + - µ µ DY σ m =.7% Z L =. Entries Mean 69.4 RM.6 on-fluctuating mass spectra: (stat. much larger than expected) generated and reconstructed ( TeV: ~ limit of CDF/D mass reach; Ldt =. fb - : a few days of LHC low-luminosity running) Realistic mass spectra: two typical MC experiments B-only fit µ µ mass (GeV) µ µ mass (GeV) +B fit Physics at LHC, -7 July 4 8

9 Fit results: m and Γ ignal mean mass: close to true value, small spread. Measured with precision of 5% (at high masses and discovery limit) or better umber of experiments TeV Z ψ,. fb TeV Z ψ, Ldt =. fb - Mean mass = 988 GeV; RM = 5 GeV umber of experiments TeV Z M, fb TeV Z M, Ldt = fb - Mean mass = 95 GeV; RM = 89 GeV m (GeV) m (GeV) (till have to account for the radiative tail in the mass distribution in the fits) ignal Γ: hard to reconstruct (FWHM dominated by resolution smearing) Physics at LHC, -7 July 4 9

10 ignificance estimators (I) Use likelihood-ratio estimator L to calculate significance of an observed signal : = ln( L L ), + B (previously used in CM by Karlsruhe group in H 4µ studies) L B where L +B is the maximum likelihood from the signal-plus-background fit (p), L B is the maximum likelihood from the background-only fit (p b ). Justification: In the large-statistics limit, L is expected to follow a χ -distribution with ndof equal to the difference in the number of free parameters between +B and B-only hypotheses (theorem proved by.. Wilks in 98). If ndof is one, then distribution of L is a standard Gaussian. Therefore, L gives the probability (expressed in number of σ s) that the pure background fakes a signal (i.e. significance). Dedicated MC study showed that L behaves as desired in a small stat. regime Physics at LHC, -7 July 4

11 ignificance estimators (II) For comparison with L, also try a few other commonly used ( counting ) estimators: =, c c c = = ( B + + B B, B Usual convention: > 5 is necessary to establish a discovery (probability of.9-7 that the pure background would mimic a signal) ), (proposed by. Bityukov and. Krasnikov, Mod. Phys. Lett. A (998) 5) cl = ln ( + B + ) exp( )). B and B number of signal and background events within m ±σ. Physics at LHC, -7 July 4

12 Z ψ at TeV,. fb - : significance s b c_rec Entries 5 Mean 5.98 RM 5.4 Underflow Overflow χ / ndf.5 / 9 Prob.89 Constant.46 ±.48 Mean 7.68 ±.9 igma 6.87 ± cl_rec Entries 5 Mean 6.46 RM.66 Underflow Overflow χ / ndf 6.98 / Prob.8589 Constant.84 ±.45 Mean ±.7 igma.54 ±.9 Fix both m and Γ to true values to calculate L cl c >> L c_rec Entries 5 Mean RM.874 Underflow Overflow χ / ndf 6.8 / 8 Prob.68 Constant.85 ±.687 Mean ±.78 igma.889 ±.6 ( s + b b ) c_rec Entries 5 Mean.89 RM.44 Underflow Overflow χ / ndf / 5 Prob.86 Constant ±.8 Mean.85 ±.88 igma.446 ± s s + b c < L c L (in agreement with other Z models, masses, luminosities tried) L_rec Entries 5 Mean 6. RM.7 Underflow Overflow χ / ndf / Prob.88 Constant.4 ±.55 Mean 6.5 ±.7 igma.85 ±.68 L In this example, obtain < L > = 6.. For. fb -, average L is more than 5 at TeV for all the Z models considered. L scales very nicely with. Ldt Physics at LHC, -7 July 4

13 Z µ + µ : CM discovery potential ) - Int. luminosity (fb Z µ + - µ : 5σ significance curves Z ψ Z η Z χ Z LRM Z M Z ALRM Z µ + µ mass reach: > TeV with. fb TeV with fb TeV with fb Z mass (TeV).B.: syst. uncertainties are not taken into account Perfect alignment, calibration, B field, etc.; Background shape, functional forms of pdf s, mass resolution perfectly known. Physics at LHC, -7 July 4

14 ummary We have set up a procedure for studying and quantifying the Z mass reach. Main points: Full simulation (CMIM) and reconstruction (ORCA) as input; Unbinned M.L. fits exploring signal and background shapes only; Likelihood-ratio significance estimator, shown to perform as desired. We have obtained mass reach estimates in Z µ + µ channel, with systematic uncertainties yet to be accounted for: More than 5σ significance above TeV at the earliest stages of datataking; Average 5σ significance at a mass of.4-4. TeV (depending on the model) with an integrated luminosity of fb -. Consistent with earlier results, but obtained with above improvements ext step: evaluate systematic uncertainties and their impact. Physics at LHC, -7 July 4 4