Higgs cross-sections

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1 Ph.D. Detailed Research Project Search for a Standard Model Higgs boson in the H ZZ ( ) 4l decay channel at the ATLAS Experiment at Cern Ph.D. Candidate: Giacomo Artoni Supervisor: Prof. Carlo Dionisi, Dott. Stefano Giagu Università di Roma La Sapienza Dottorato in Fisica, XV Ciclo Introduction The Large Hadron Collider at CERN [] is a proton-proton collider that has been working at s = 7 TeV from March 00 and will continue to acquire data until the end of 0. So far, as it is possible to see in Fig.a, a total luminosity of about.7 fb has been delivered by the accelerator. The research project I will present here is taking place within the ATLAS (A Toroidal LHC ApparatuS) Collaboration []. ATLAS is a general purpose experiment which has been conceived to cover many possible scenarios for particle physics; so far, with an efficiency of 95%, it has collected about.5 fb and has already showed many interesting results on many different subjects. In this research project I will first focus on the Higgs boson and how it can be detected at a hadron collider, then I will briefly show which sub-detectors in ATLAS are fundamental for the reconstruction of muons and electrons which are used in this Higgs search; the last part of this research project will show the results that we have already obtained and all the improvements we want to include in the forthcoming analyses in order to improve our Higgs discovery sensitivity.! H!!!: rare channel, but the best for low mass! H!WW (*) : 3.5 ATLAS Online Luminosity s = 7 TeV 3 LHC Delivered!!l!l!: very important in the ATLAS Recorded intermediate mass range.5!! l!qq: Total Delivered: highest.68 fbrate, Total Recorded:.55 fb important at high mass! H!ZZ.5 (*) : Total Integrated Luminosity [fb ] Higgs cross-sections!! 4l: golden channel!! ll!!: good for high mass 0.5!! llbb: also high mass! H!"": 0 0/03 0/04 good 0/05 signal/ 0/06 0/07 3/07 3/08 background, important at low Day in 0 mass, rare (a) -./0$/34/5/6$7$8/$4976:5/6$;<=$>?!$@ (b) A!$! Associated prod. H! bb-bar!! tth, WH, ZH " #$%&'$ ((!)*)*$ ++!,)$ --$ Figure : Luminosity delivered by the LHC and acquired by ATLAS during 0 (a) and Higgs boson production! It is useful cross for section the discovery times branching ratio!"#$ for all decay channels!"%$ (b).!&'$ ()$! It is very important for Higgs!'#$ )*#$ (&+$!+$ property studies if SM Higgs is discovered )##$,*$ )&,$ #&#($ >B!!$A$,C""C!!$ D&$E<FGH$I<JJ$/F95=/$<0$DK>DL$./0.$ '$

2 The Higgs Boson and the H ZZ l + l l + l decay channel The Standard Model (SM) [3] of particle physics is currently our best (mathematical) description of the sub-atomic world. This description agrees with all experimental results gathered until now and this has brought the scientific community to strongly believe that the SM represents an important step towards an unified theory of all known interactions. Among its predictions, there is the presence of a particle called the Higgs boson which has not been seen yet and its mass is a free parameter in the theory. The Higgs boson presence in the theory is required in order to be able to assign masses to the fermions and to the gauge bosons. In particular, the coupling of the Higgs to a particle is proportional to the mass of that particle and thus its branching ratio will be dominated by the decays in W or Z pairs for high Higgs masses. This is clearly visible in Fig.b where, in light blue, it is plotted the Higgs cross section times the branching ratio of the decay H ZZ 4l (l = e, µ). This decay channel is the most suppressed one at high Higgs masses, and nevertheless it is always referred to as the gold-plated channel. This is because electrons and muons (unlike jets and missing transverse energy) represent the cleanest signatures at a hadron collider and through this it is possible to recover for the small amount of events. Another fundamental aspect is that, in this channel, it is possible to reconstruct completely the decay; this means that it will be possible to measure the Higgs mass and also its spin. Thus this channel proves to be fundamental both for a discovery and for precision measurements of the Higgs fundamental quantities. The ATLAS Detector As already said, ATLAS is a multi-purpose apparatus with cylindrical geometry. Tracking information is provided by the Inner Detector (ID) [4], which is composed of a silicon pixel detector, a silicon microstrip detector and a transition radiation tracker; these three sub-detectors are surrounded by a solenoid granting a T magnetic field. Electromagnetic showers are measured in a lead-liquid-argon (LAr) sampling calorimeter with high granularity [5]. This calorimetric system is then surrounded by the muon spectrometer (MS), which is instrumented with three superconducting toroids and precision tracking chambers [6]. All these sub-detectors are fundamental for the H ZZ l + l l + l decay channel; electrons are identified by an electromagnetic cluster associated to a track and muons are reconstructed combining a track from the MS with the matching track from the ID. Search for the Standard Model Higgs boson in the decay channel H ZZ 4l (l = e, µ) Here it will be presented the latest ATLAS result on this search using.8 fb for the 4µ channel,.96 fb for the eµ channel and.98 fb for the 4e channel. Event Selection The gold-plated channel provides a very clean signal at a hadron collider and this is reflected in the simple event selection that we apply: Trigger: Single-lepton triggers (thresholds E T > 0 GeV for electrons and p T > 8 GeV for muons) which grant 99.8% efficiency on our signal; Flavor/Isolation: Each candidate must have two same-flavor, opposite charge lepton pairs and all these leptons must be isolated; Primary Z: One of the two lepton pairs must reconstruct the Z mass within a 5 GeV.

3 Backgrounds The relevant backgrounds for this analysis are the ZZ( ), the Z + jets and the t t production. These last two are considerably decreased by our analysis selections, for example by the isolation and mass requirements; the ZZ( ), instead, provides the same exact signature of our signal and it is thus referred to as an irreducible background. ZZ( ): this background, which is the dominant one, has been estimated using Monte Carlo simulation and has been assigned a systematic uncertainty of 5%. Z + jets: its contribution is very small and it has been estimated extrapolating from a control region (Z with another lepton pair without impact parameter and isolation requirements) to our signal region; this extrapolation induces a systematic uncertainty between 0% and 40% (the difference is due to the difference between the Z produced in association with light flavor or heavy flavor jets). t t: the yield of this background has been taken from Monte Carlo predictions and it has been verified on a control sample of opposite sign electron-muon pairs consistent with the Z boson mass. The total number of expected events for the background is 8 4 and a total of 7 events (6ee, 9eµ and µµ) have been observed on data on the whole mass range; the final plot after all the analysis cuts can be seen in Fig.. H!ZZ (*)!4l!"#!$%! A total of 7 events are selected by the Figure : Distribution of m 4l analysis of thealgorithm: selected candidates 6ee, 9eµ, µµ with comparison to the background Invariant expectations. mass leading The (top) signal and expectation! Expected: for 84 = 50, 0, 480 GeV is also shown. sub-leading (bottom) lepton pair!"##$%$&'(('##$ *+$,-./0$-33.$.4/5674.$-8$*9!*:$ ()$ Limits Using the collected data, it is possible to set upper limits on the Higgs boson production cross section at 95% CL, with the CL s modified frequentist formalism with the prole likelihood test statistic; the test statistic is 55 evaluated with a maximum likelihood t of signal and background models to data. Fig.3 shows the expected and observed exclusions as a function of the Higgs mass. The consistency with the background-only hypothesis is quantied using the p-value, the probability that a background-only experiment will uctuate more than a given observation. The most signicant deviation from the background-only hypothesis is observed at = 4 GeV with a p-value of 6.8%. The Standard Model Higgs boson is excluded at 95% CL in the ranges and 4 6 GeV. This is the latest public ATLAS result shown at a conference and it includes only the H ZZ 4l analysis; if we combine all decay channels (result shown at the EPS conference, see [7]) we obtain the results in Fig.4. It is clearly visible in Fig.4a 3

4 (a) (b) 95% CL limit on σ/σ SM 0 0 ATLAS (*) (*) H ZZ 4l 4l Ldt = fb fb s=7 TeV Observed CL s Expected CL s σ σ 95% CL limit on σ/σ SM 0 0 ATLAS (*) (*) H ZZ 4l 4l Ldt = fb fb s=7 TeV Observed CL s Expected CL s σ σ (c) (a) (d) (b) Figure 3: Observed and expected exclusions as a function of for 0 < < 00 GeV in Figure A.: Expected (dashed) and observed (full line) 95% CL upperlimittothestandardmodelhiggsbosoncrosssection as a function Fig.3a of and the 00 Higgs < boson < mass, 600expressed GeV in as Fig.3b a ratio of the Standard Model cross section. 9 95% CL limit on / SM 0 Exp. Obs. Exp. Obs. H H (.08 (.08 fb ) fb ) H ZZ H ZZ llll llll ( fb fb) ) H WW H WW l l l l (.70 (.70 fb fb) ) H ZZ H ZZ llqq llqq (.04 (.04 fb fb) ) W/Z H, W/Z H H, H bb bb (.04 (.04 fb fb) ) H ZZ H ZZ ll ll (.04 (.04 fb fb) ) H H (.06 (.06 fb ) fb ) 95% CL Limit on / SM 0 ATLAS Preliminary Observed Expected CLs Limits Ldt =.0-.3 fb s = 7 TeV ATLAS Preliminary L dt ~.0-.3 fb, s=7 TeV CLs limits (a) (b) Figure 4: Observed and expected exclusions as a function of for all different channels in Fig.4a and combined in Fig.4b, as shown at the EPS conference. 4

5 that in the low mass region the H W W lνlν is the most important channel and the H ZZ 4l comes immediately after that (but we must remember that the best precision on the Higgs mass measurement will by far be achieved by our channel). In the high mass region the most important analysis is H ZZ llνν but between 00 and 300 GeV our decay channel is the most sensitive. Analysis for the 0 dataset All the results that have been shown in the previous lines indicate that the search for the Standard Model Higgs boson is not completed yet; the mass region below 40 GeV has not been excluded yet and also the region around 50 GeV needs further studies. Statistics: Our analysis is substantially limited by the statistics acquired so far and thus we simply need more statistics to have better limits. If the LHC continues to deliver an instantaneous luminosity of 0 33 cm s we can expect to have, by the end of 0, at least 6 fb. With such a sample the H ZZ l + l l + l decay channel alone will be able to exclude the presence of a Higgs boson within 00 GeV and 300 GeV (or discover it with more than 3 σ) and also it will be a very helpful channel in the low mass scenario. The expected sensitivity to a 95% CL exclusion, a 3σ or 5σ discovery are shown in Fig.5 as a function of the Higgs mass and of the recorded luminosity. At the moment it is not simple to understand how much statistic will be available by the end of my Ph.D. program; 6 fb appear to be a low estimation and it is also possible that we will have 7 fb. Anyway, our goal is to use this gold plated channel to exclude or discover the Higgs on the whole mass range and, if it happens to be found, to be the first to measure with good precision the Higgs mass and spin. In case of a Standard Model Higgs exclusion at high masses, it will be possible to reduce the upper limit of the cross section of a non-standard Higgs decaying in ZZ. Integrated Luminosity, fb ATLAS Preliminary (Simulation) 5 s=7 TeV 5 s=8 TeV 3 s=7 TeV 3 s=8 TeV 95% CL s=7 TeV 95% CL s=8 TeV Figure 5: Luminosity required to give exclusion, evidence or discovery sensitivity for a Standard Model Higgs with data at s = 7 or 8 T ev. Object Reconstruction: Introducing electrons re-fitted for the bremsstrahlung energy loss can provide a gain, as first studies on these objects show, around % on our signal events and the resolution on the impact parameter will be significantly improved. Lepton Acceptance: Another important point where there is something that can be still gained is the acceptance of the muons and the electrons; so far we have been using only leptons with tracking information, but the ID is limited in η (<.5) and thus we are not using muons and electrons with η >.5. I will introduce in the analysis muons with a 5

6 track only in the MS or electrons with only clusters in the forward calorimeters; in this way the composition of our background will change and I will study it. Trigger: So far only single lepton thresholds have been used in the analysis but I will add to these also triggers requiring two leptons, which can grant us lower thresholds and thus give us the chance of increasing the number of our signal events. Kinematic selections: The selections concerning primary Z and secondary Z masses, as well as the minimal E T of the leptons, need to be re-optimized, especially for a light Higgs; I have performed this study and shown that such an optimization can improve our sensitivity. Angular Distributions: There are four decay angles and one production angle that show differences between our signal and our irreducible background (see [8]). This is because the spin correlations are different between q q ZZ( ) 4l and H ZZ( ) 4l since we are looking for the Standard Model Higgs boson, which is a scalar. A maximum likelihood fit using these correlations can improve our discovery or exclusion potential, especially for > 00 GeV. We must note, once again, that only this channel can provide such a measurement since all objects in the Higgs decay are reconstructed. Multivariate analysis: Along with the cut based analysis, I will develop a multivariate analysis in order to exploit our informations on the signal and the backgrounds as much as it is possible. For example we cannot apply a cut on the angular distributions, but if we put them in a multivariate discriminator we can exploit all the correlations and improve our analysis. References [] LHC Conceptual Design Report, CERN/AC/95-05 [] Atlas Collaboration, ATLAS Detector and Physics Performance Technical Design Report, CERN/LHCC 994/5 (999) [3] S. L. Glashow, Partial symmetries of weak interactions, Nucl. Phys. (96); S. Weinberg, A model of leptons, Phys. Rev. Lett. 9 (967) 64; A. Salam, Weak and Electromagnetic Interactions, Proceedings of 8th Nobel Symposium on Elementary Particle Theory Relativistic Groups and Analitics, Stockholm, Sweden, 968; P. W. Higgs, Broken symmetries, massless particles and gauge fields, Phys. Rev. Lett. (964) [4] ATLAS Collaboration, Inner Detector Technical Design Report Vol. I and II, CERN/LHCC 976 (997) [5] ATLAS Collaboration, Liquid Argon Calorimeter Technical Design Report, CERN/LHCC 96-4 (996) [6] ATLAS Collaboration, Muon Spectrometer Technical Design Report, CERN/LHCC 97- (997) [7] ATLAS Collaboration, Update of the Combination of Higgs Boson Searches in.0 to.3 fb of pp Collisions Data Taken at s = 7 T ev with the ATLAS Experiment at the LHC, ATLAS-CONF-035 (0) [8] J. S. Gainer, K. Kumar, I. Low, R. Vega-Morales, Improving the sensitivity of Higgs boson searches in the golden channel, arxiv:08.74v (0) 6