1 Research Project

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Georg-August-Universität Göttingen University of Pisa

Annual Report

Report Period: April 2015 - April 2016

PhD Year: 1 X 2 3 4 ≥ 4

PhD Supervisor / Working Group: Prof. A. Quadt, Prof. V. Cavasinni Thesis Committee: 1. Prof. A. Quadt

2. Prof. S. Lai 3. Prof. V. Cavasinni

by

Antonio De Maria

ademari@phys.uni-goettingen.de

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1 Research Project

1.1 PhD Topic:

The main topic of my PhD is the study of the Higgs Boson decay in the di-tau channel. In particular, most of the effort now is focusing on the coupling measurement using the new Run2 data. However, this first year was mainly dedicated to the Qualification task (which will be discussed in the subsection 1.3.1) and to some preparatory/related studies to the main topic, which will be also discussed. In the following section, an introduction to the main topic will be given focusing mainly on ATLAS experiment only results.

1.1.1 Motivation and Outline

One of the goals of LHC is the investigation of the origin of the electroweak symmetry breaking and the experimental confirmation of the Brout-Englert-Higgs mechanism [1]- [6]. With the discovery of a Higgs boson in 2012 an important milestone has been reached.

The main production mechanisms available for a SM Higgs boson are, ranked from the highest to lowest cross section: gluon fusion (ggF ) [7], vector boson fusion (VBF ) [8] and associated production with a vector boson (VH ). The measurements rely principally on studies of the bosonic decay modes, H → γγ, H → ZZ^∗ and H → W W^∗. In order to establish the mass generation for fermions as implemented in the SM, it is of prime importance to demonstrate the direct coupling of the Higgs boson to fermions and its proportionality to mass [9]. The most prominent candidate decay modes are the decay into τ leptons, H → τ τ , and bottom quarks, H → b¯b.

More favourable signal-to-background conditions are expected for H → τ τ decays, due to the distinct H → τ τ final state topologies and excellent τ reconstruction and identification in the ATLAS detector.

The analysis of the H → τ τ decay mode started in 2006 using Monte Carlo samples; then it has been continued using 2010 data at a centre-of-mass energy of √

s = 7 TeV and 2011/2012 data at √

s = 8 TeV. The results reported in this section are based on the full proton-proton dataset collected by the ATLAS experiment, corresponding to integrated luminosities of 4.5 fb⁻¹ at√

s = 7 TeV and 20.3 fb⁻¹at√

s = 8 TeV. All combinations of leptonic and hadronic τ decays are considered [11].

The various Higgs production processes lead to different final state signatures, which have been exploited by defining an event categorisation. Two dedicated categories are considered to achieve both a good signal-to-background ratio and a good resolution for the reconstruction of the τ τ invariant mass. The Vector-boson-fusion category, enriched in events produced via vector-boson fusion, is defined by the presence of two jets with a large separation in pseudo-rapidity. The Boosted category contains events with a large transverse momentum of the reconstructed Higgs boson candidate. It is dominated by events produced via gluon fusion with additional jets from gluon radiation. A multivariate analysis technique is used to extract the final results.

An excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.5 (3.5) standard deviations. This excess is consistent with resulting from H → τ τ decays with m_H = 125 GeV. The measured signal strength, normalised to the Standard Model expectation for Higgs boson of m_H = 125.36 GeV is:

µ = 1.42^+0.27_−0.26(stat.)^+0.32_−0.24(syst.) ± 0.10(theory syst.)

The signal strength is defined as the ratio of the measured signal yield to the SM expectation.

The value µ = 0 (µ = 1) correspond to the absence (presence) of a Higgs boson signal with the SM production cross section. The results obtained strongly suggests the presence of a SM Higgs

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boson which decay into a τ pair. Nevertheless, more data are needed in order to improve the found excess to a significance of 5 (or more) standard deviations. Indeed, 5 standard deviations are the benchmark of the discovery of a new particle. Actually the Run2 analysis is ongoing and one of the aims of my PhD is to develop and perform all the analysis chain focusing mainly on the H → τ τ →lep-had channel. A crucial question is whether there is only a single Higgs doublet, as postulated by the SM, or whether the Higgs sector is modified with respect to the SM, for example with a second doublet leading to more than one Higgs boson of which one has the SM-like properties, as predicted in many beyond-Standard-Model (BSM) theories [12]. The

"hierarchy problem" regarding the naturalness of the Higgs boson mass, the nature of the dark matter, and other open questions that the SM is not able to answer also motivate the possible exsistence of additional new particles and interactions.

Another important aim of the PhD is the CP measurement of the Higgs Boson. In the SM only CP-even is allowed. In order to verify the presence of CP-odd contribution, the HVV vertex in the VBF production mode could be studied. In particular, some anomalous coupling could give a CP-odd contribution and some parametrization are available to study this effect. To test the CP invariance, several CP-odd observables could be defined and computed from event kinematic variables. Then, if it is found that the mean of these observables are different from 0, then it could be concluded that there is a CP violation. Results from Run1 analysis can be found in [13]

A second method to analyse the CP nature of the Higgs Boson is based on the spin correlation between the taus produced by Higgs decay. In this case the Higgs Boson can be parametrized by a scalar/pseudo-scalar Higgs mixing angle Φ_τ, which can be determined in H → τ⁺τ⁻ decay with subsequent τ -lepton decays to charged prongs. In order to define an observable φ^∗ which is sensitive to Φ_τ, two reconstruction method can be used: the ρ-decay plane method for τ^± → ρ^± and the impact parameter method for all other major tau decay. More details on the reconstruction method and simulations results can be found in [14]. Till now, both these analysis on Higgs CP are ongoing and possible tasks inside one of them can be a really interesting starting point to be involved in.

1.1.2 Qualification Task: Development of new algorithms for the data quality evaluation

The aim of the qualification task was the development of new algorithms to judge data quality (DQ ) during the online ATLAS data taking process. The work was done under the supervision of Yosuke Takubo, the Pixel DQ convener.

The first part of the qualification task have been dedicated to study the ATLAS data taking process and the Data Quality Monitoring Framework (DQMF ), a framework which is used to judge DQ both online and offline. DQ evaluation is based on the express-express stream, which contains ' 10% of the recorded data. This stream is mainly used to have a fast evaluation of data quality and provide information for the bulk reconstruction process, the last reconstruction step for all recorded data. Moreover, a good knowledge of the Pixel detector was necessary to have a clear overview of the analysed quantities, but the developed algorithms can be used also by other detectors.

Two talks were dedicated to present DQMF and the related infrastructure:

• [15], presented during the Pixel DQ workshop held in March;

• [16], presented during the Pixel week held in April.

The second part of the qualification task was dedicated mainly to the development and test of new algorithms. At the beginning a general overview of the already used algorithms was done

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1 RESEARCH PROJECT 3

in order to both learn how to write new code and how to test it.

Every algorithm can take in input one or two histogram (the current histogram under analysis and a reference histogram) ; the output result is given by a colour flag: red (bad), yellow (warning), green (good), grey (undefined). To get the output, usually a comparison with some thresholds values is done. One central idea for the development of new algorithms was the use of multiple thresholds to get the result, e.g. passing from two thresholds to four thresholds to have a more accurate control on the quantities reported into the histograms. A talk about a new algorithm developed for this purpose and the related tests was given during a Pixel Activity

& status meeting held in May [17]. After this, a period have been dedicated to learn two test methods:

• an online method, which simulates the data taking process injecting histograms from pre- viously recorded data or simulation;

• an offline method, which takes in input the express-express stream of a specified run and re- perform the data taking process using algorithms and quantities specified in a configuration file.

Both methods have been used during the first tests and then it was chosen to use only the offline method, since it is faster and easily manageable (ca. 1 hour from starting to write new algorithm code to the fist test results). An accurate overview of the new developed algorithms and their related tests was given during the Pixel week held in November [18].

Finally, to summarise what has been done during this year, a good knowledge of the Atlas detector and data-taking process have been acquired; moreover, new algorithms for the DQ have been developed and successfully tested.

1.1.3 Missing Mass Calculator as a technique to reconstruct the mass of resonances decaying into tau pairs

An accurate reconstruction of a resonance mass decaying into a pair of tau leptons is a difficult task because of the presence of multiple undetected neutrinos from the tau decays. The Missing Mass Calculator (MMC) is a sophisticated method to optimise the reconstruction of this events.

It is based on the requirement that mutual orientations of the neutrinos and other decay products are consistent with the mass and decay kinematics of a tau lepton. This is achieved by minimizing a likelihood function defined in the cinematically allowed phase space region. MMC was one of the most powerful tools used in SM-Higgs to tau tau searches in Run1 at LHC. Now, in Run2, LHC collides proton-proton at center of mass energy √

s = 13 TeV and at higher luminosity.

Therefore, many efforts need to be done to optimise the analysis tools to the new experimental conditions. Amongst these tools, MMC requires to be retuned in order to play a key role again in the searches of the Higgs boson in di-tau final states. During this year, I was involved into the MMC retuning for the H → τ τ → hh channel. Results of many studies were presented in the Mass Task Force weekly meetings, who where chaired by Dr. A. Pranko, one of the main author of the MMC algorithm. Also, lots of feedback about these studies have been received during the weekly Goettingen Higgs meetings. The retuning procedure has been tested really in deep and the final results are comparable with Run1 results. Finally, a talk about MMC retuning studies has been given at the Deutschen Physikalichen Gesellschaft (DPG) Spring Conference held from 29/02/2016 to 04/03/2016 in Hamburg [19].

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1.1.4 Tau fake rate measurement in Z → µµ + jets events

The main objects of this task are the jets which fake taus. Thus it was chosen a channel on which this type of study can be performed in a clear way. The final aim is the measurement of the jet-to-tau fake rate and scale factors in terms of tau candidate ID, p_T, η and prongness.

Other secondary studies performed on the same channel are the measurement of the MET resolution, study the fake modelling of tau-substructure observables. Study of the MET resolution can be performed given the absence un neutrinos into the final-state; thus we can study detector performance related to MET reconstruction. The tau-substructure is a new algorithm for tau reconstruction in the ATLAS experiment and it was developed for Run II. Actually many efforts are ongoing to have a fully integration of this reconstruction algorithm at analysis level (systematics, performance measurement, etc). The analysis was done using ntuples produced with a dedicated code in the xTauFramework, [20]. Further steps of the analysis have been performed using the xTauAna plotting framework. Using a similar infrastructure like the one used into the analysis of H → τ τ → lh channel gave me the possibility to be frequently both developer and tester of the both frameworks. Thus lead us to be always synchronized with main analysis in the HLepton group. A JIRA page was also maintained to document these studies:

https://its.cern.ch/jira/browse/ATLHLEPTON-74.

Moreover, results of this analysis were often presented in the Goettingen Higgs meetings. Like in the case of MMC studies, also in this case many feedbacks/discussions lead to improvement of the results.

1.1.5 Outlook and Plans for the Next 12 Months

To summarise this section, lots of activities have been done during this year. Apart from qualification task, which ended successfully in February, other activities are still ongoing and will be finalize during next year. H → τ τ → lh analysis will be the main item, since lots of optimisa- tion studies need to be performed; of course Run I experience is a solid baseline, but the many possibilities to improve results have been proposed.

2 Teaching

List of teaching activities:

• Physik IV Ubung (Prof. A. Quadt)

• Bachelor Physikalisches Fortgeschrittenenpraktikum (Prof. A. Quadt)

3 Lectures

List of the attended lectures:

• Physics of the Higgs boson and Electroweak Symmetry Breaking (Prof. S. Lai)

• Hadron Collider Physics (Prof. S. Lai)

• Statistical Methods of Data Analysis (Prof. S. Lai)

• Physics beyond the Standard Model (Prof. S. Lai)

• Grundkurs A1.1. (Prof. K. Paepke) (considered as teaching activity for GAUSS credits)

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4 GIVEN TALKS 5

• Grundkurs A1.2. (Prof. K. Paepke) GAUSS requirements have been fulfilled.

List of the attended seminar:

• Mitarbeiterseminar during Summer semester 2015

• Mitarbeiterseminar during Winter semester 2015/2016

4 Given Talks

List of talks/posters given in local meetings, collaboration meetings, workshops and conferences:

Meeting Type of Talk Number of talks

AG Quadt/AG Lai Group Meet- ing Göttingen

PhD Status report 2

HLepton Meeting Lep-Had studies 3

Higgs Group Meeting Göttingen Fake Rate and MMC studies 12

Mass Task Force meeting MMC studies 16

Hardware Group Meeting Göttin- gen

Qual. Task Status 4

Pixel week Qual. Task and Egamma Perf.

studies

2

Pixel Offline Software Meeting Egamma Perf. studies 2

Pixel DQ Workshop Qual. Task studies 1

Data Quality Meeting Data Quality report (class 2 shift) 3

5 Attended Schools and Conferences

From 29/02/2016 to 04/03/2016, I joined the DPG Spring Conference in Hamburg. A talk has been given regarding MMC retuning studies (see sec. 1.1.3.). The conference was really interesting and gave me the opportunity to have a complete overview of the various fields of Physics (Astronomy, Matter Physics, etc.). Also, I have the opportunity to meet and discuss with lots of people which are collaborating to the various H → τ τ analyses.

6 OTP Shifts

OTP Shifts have been done in the contest of the Pixel Detector:

• class 2 shift: Pixel offline DQ. This shift takes role of checking the data quality taken with Pixel detector at the point of calibration loop of the data processing before bulk processing.

The results of quality check in this shift is reflected to the bulk processing, so that it takes important role in data reconstruction.

• class 3 shift: Study of the effects of Pixel detector increasingly masking on egamma performance. The aim of this project is to check the impact of different layer mask combination on electron reconstruction. The project is done under the supervision of Yosuke Takubo and in close collaboration with Daiki Yamaguchi. The analysis strategy was discussed and fixed together, giving me the possibility to experiment new ideas and suggest futher developments. The major steps of this project are:

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– apply different Pixel detector masks in the Athena reconstruction algorithm. This allows to produce modified xAOD samples which can be analysed to extract results of the masking procedure;

– analysis of the samples produces in the step before. To do this, a xAOD analysis code as been developed and tested. A description of these studies can be found in [21]- [22].

Class 1 / Class 2 Class 3

Obligation 11.24 0.24

Done 14 0.13

Difference (Done-Obligatory) -2.76 0.11

Table 1: Previous Year (Finished)

Class 1 / Class 2 Class 3 Obligation

Done

Difference (Done-Obligatory)

Table 2: Present Year (Ongoing)

7 Outreach

Talking about outreach activities, I was involved into stand construction for the Ideen Expo 15, [23]. This was most probably the first time I joined in a so big exhibition outside a physics institute. It was a good opportunity to understand how Physics can be shared among different people guided only by their curiosity. Always on the spirit of sharing Physics with young students, I joined also the ROOT tutorial during the Hasco Summer School held in Goettingen, [24]. Also this was a good opportunity to have a comparison with students from different countries and share with them many ideas on High Energy Physics.

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REFERENCES 7

References

[1] F. Englert and R. Brout, Broken symmetry and the mass of gauge vector mesons, Phys. Rev.

Lett. 13 (1964) 321.

[2] P. W. Higgs, Broken symmetries, massless particles and gauge fields, Phys. Lett. 12 (1964) 132.

[3] P. W. Higgs, Broken symmetries and the masses of gauge bosons, Phys. Rev. Lett. 13 (1964) 508.

[4] G. S. Guralnik, C. R. Hagen, and T. W. B. Kibble, Global conservation laws and massless particles, Phys. Rev. Lett. 13 (1964) 585.

[5] P. W. Higgs, Spontaneous symmetry breakdown without massless bosons, Phys. Rev. 145 (1966) 1156.

[6] T. W. B. Kibble, Symmetry breaking in non-Abelian gauge theories, Phys. Rev. 155 (1967) 1554.

[7] S. Alioli et al., NLO Higgs boson production via gluon fusion matched with shower in Powheg, JHEP 0904 (2009) 002, [arXiv:0812.0578].

[8] P. Nason and C. Oleari, NLO Higgs boson production via vector-boson fusion matched with shower in Powheg, JHEP 1002 (2010) 037, [arXiv:0911.5299].

[9] S. Weinberg,A Model of Leptons, Phys. Rev. Lett. 19 (1967) 1264.

[10] J. Beringer et al. (Particle Data Group), Phys. Rev. D86, 010001 (2012) , http://pdg.lbl.gov.

[11] ATLAS collaboration, Evidence for Higgs boson Yukawa couplings in the H → τ τ decay mode with the ATLAS detector, ATLAS-CONF-2013-064.

[12] ATLAS collaboration, Prospects for New Physics in Higgs Couplings Studies with the ATLAS Detector at the HL-LHC, ATL-PHYS-PUB-2014-017.

[13] http://arxiv.org/pdf/1602.04516v1.pdf [14] http://arxiv.org/abs/1510.03850

[15] https : //indico.cern.ch/event/377208/contribution/5/attachments /752129/1031823/slides.pdf

[16] https : //indico.cern.ch/event/380923/session/6/contribution/4/attachments /758847/1040925/DQM − 30042015.pdf

[17] https : //indico.cern.ch/event/397242/contribution/2/attachments /796380/1091562/M ultipleT hresholdAlgorithms.pdf

[18] https : //indico.cern.ch/event/405685/session/3/contribution/15/attachments /1185532/1718524/presentation − 11112015.pdf

[19] https : //indico.cern.ch/event/485903/contributions/1163838/attachments /1228058/1799002/talk_new.pdf

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[20] https : //twiki.cern.ch/twiki/bin/viewauth/AtlasP rotected/XT auAnalysisF ramework

[21] https : //indico.cern.ch/event/497385/contributions/2010258/attachments/1257924/1857785/slides_egamma0414.pdf [22] https : //indico.cern.ch/event/496456/contributions/2016536/attachments/1228707/1800204/slides_egamma₀215.pdf [23] http : //www.ideenexpo.de/IdeenExpo₂015/20150712_Abschluss_IE2015.php

[24] http : //indico.cern.ch/event/388801/timetable/20150720

1 Research Project

Contents

1 Research Project

2 Teaching

3 Lectures

4 Given Talks

5 Attended Schools and Conferences

6 OTP Shifts

7 Outreach

References