My work for HERA, LHC and ILC.

Tomáš Laštovička

Tuesday seminarNovember 27, 2007

Tomáš Laštovička 2

Introduction

I have joined LCFI group at Oxford in May 2007

PhD study: DESY – H1 Experiment where I finished my PhD – Humboldt University, Berlin

Postdocs: DESY (2004) CERN fellowship (2005-2007) – LHCb Experiment 05/2007+ University of Oxford

Keywords: silicon vertex detectors, QCD, structure functions, tracking, vertexing, programming, …

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DESY and the H1 Experiment( ? – 2004 )

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DESY – H1 Experiment

When deciding about master thesis topic at Charles University, Prague, I opted for DESY This actually started my low Q2 (and low x) precision measurements

period. After finishing thesis I decided to continue the business and moved to

DESY Zeuthen close to Berlin (still being student at Charles University, for some time).

H1

DESY Hamburg

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Low Q2 inclusive cross sections

Member of ‘ELAN’ group – Inclusive Measurements and QCD fits …which I had pleasure to convene in 2002-2005.

Focused on measurements of F2 and FL proton structure functions Steep rise towards low Bjorken x is one of few HERA discoveries

F2 is not calculable from the first principles Related to quark densities, in the leading order as

1993 2000

quarks

iii qqxeF )(22

Turn-over attributed to FL

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low Q2 1999 dedicated run

Transition region between perturbative and non-perturbative kinematic range

shifted vertex 2000 run

Low Q2 x-section and F2

determination at low x

Situation in 2002 (now even better coverage)

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Key components I

Backward Silicon Tracker

Shifted vertex

SpaghettiCalorimeter

e+ pNominal vertex

~70cm

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Key components II

Backward Silicon Tracker (BST) scattered electron measurement 95% track reconstruction efficiency 20μm resolution 8 planes, 16 segments each

e+

SpaCal (Spaghetti Calorimeter) 1192 square cells, lead-scintillating fibers

0.2% calibr. precision at 27.6GeV High efficiency L1 (LHCb’s L0) trigger

(>99%) x-y view:

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Analyses

>95% of analysis time was devoted to understanding and description of detector components Detector calibration (SpaCal - electron, LAr - hadrons) Alignment (BST, SpaCal, BDC) Efficiency (BST) – quite a challenge …

It was literally “pushing the limits” style of analysis leading to some novel approaches, e.g.

F2 measurement from ISR events without measuring the scattered electron

FL determination at Q2 ~ 1GeV2 and below (next slide) - this region is too non-perturbative even for F2, not mentioning FL

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FL in the transition region

Status of 2003 preliminary analysis shown.

FL was actually never measured directly at HERA low proton beam energy runs in 2007 (preliminary analysis in 2008?)

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Contribution to phenomenology

Attempt to describe internal proton structure as a fractal:

…but much more colorful, of course! (QCD)

Use concept of fractal dimensions, estimate PDFs and fit F2.

and most enjoyful time I had in Physics so far…

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Contribution to phenomenology

This worked surprisingly well leading to lowest χ2 fit of F2 data and it is going to be used in the forthcoming H1 publication as well.

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CERN and the LHCb Experiment( 2005 – 2007 )

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CERN fellowship

In 2004 I was awarded CERN research fellow position and joined the LHCb Experiment in February 2005

I started with Metrology/Alignment of VELO (Vertex Locator) telescope VELO-only fast simulations (rest of LHCb subdetectors not simulated)

Then I directed myself more into VELO-only reconstruction of generic tracks/vertices and its applications Generic pattern recognition – PatVeloGeneric class and Generic Vertex finding and reconstruction Applications:

alignment issues, open velo, luminosity measurements, test-beam data, …

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Vertex Locator (VELO)

21 tracking stations on two sides 42 modules, 84 sensors plus pile-up sensors

Optimised for tracking of particles originating

from beam-beam interactions fast online 2D (R-z) tracking fast offline 3D tracking in two

steps (R-z then phi-z)

Velo halves are moved from the LHC beam by 30mm during the beam injection and tuning.

nominal vertex area

pile-up veto sensors

R sensors

φ sensors

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Open VELO tracking – aperture 30mm

54 μm69 μm

Ntr ≥ 10 Ntr ≥ 10

Ntr ≥ 4 Ntr ≥ 4

83 μm108 μm

Designed beam-spot size: 70μm (100μm beam profiles)

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Luminosity measurements

Reconstruction of beam parameters from beam-gas interactions in order to calculate beam overlap integral and thus luminosity. It was my pleasure to lead luminosity measurements sub-group (a

part of Production and decay models WG)

Ntr > 14

Xenon simulation

~18μm

x y

Designed beam-spot size: 70μm (100μm beam profiles)

VELO

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Possible application

Measure Z0→µ+µ- cross section as a function of rapidity (and scale, eventually).

Use the data in QCD fits to pin down proton PDFs at high scales but low x Drell-Yan pair production is

included in most of QCD fitters.

Compare with predictions without fitting the data… Cross-check: it simply must

agree (or there is a problem).

Presented at HERA-LHC workshop in 2006.

Z0→µ+µ-

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VELO in test-beam I Successful application of generic PR/vertexing algorithms on real

VELO data (10 sensors mounted, 6 read out at once…)

allowed to see beam position from interactions in VELO tank, targets and in sensors themselves…

…which made minimum of 5 people happy and showing teeth:

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VELO in test-beam II

Panoramix display of typical VELO test-beam tracks with targets vertices not shown but reconstructed.

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University of Oxford – LCFI group (ILC)( May 2005 till now )

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LCFI = Linear Collider Flavour Identification

I am involved in:

Physics

Higgs self-coupling: ZHH channel

SUSY: sbottom Decay Analysis

Technical tasks

Kalman Filter for Vertex Fitting

Jet Tagging Algorithms (NNs, Boosted Decision Trees)

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Higgs Self-coupling? Higgs Potential

To experimentally determine the shape of the Higgs potential the self-coupling of the Higgs field must be measured

In Standard Model , independent measurement may reveal an extended nature of the Higgs sector:

as measured by λHHH

Higgs potential from MH

some other completely fictional potential from an extended model

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Why ZHH channel?

Measurement of cross section gives a handle to measure the Higgs self-coupling constant.

Benchmark channel for ILC. To evaluate various detector concepts. Highly non-trivial to analyse.

Another option is WW fusion channel. Small cross section (500GeV ILC).

Roughly Δλ/λ ~ 1.75*Δσ/σ

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SUSY and Cosmology

There is 23% of Cold Dark Matter in Universe – as measurements tell us.

Neutralino is Dark Matter candidate. During Universe expansion at some point

supersymmetric particles are no longer produced but the existing ones may annihilate – the rate can be calculated.

In most of the SUSY parameter space there are still too many neutrinos left.

Cold Dark Matter favors some particular SUSY scenarios.

For effective co-annihilation of particles the mass splitting should be small – leading to small energies of visible particles.

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sbottom and neutralino

If sbottom (stop) and neutralino have a small mass split they can account for co-annihilation in early Universe through this type of diagrams:

Sbottom can be produced at ILC, then it decays to b and neutralino:

b~

0~b

,Z

b~

b~

0~t

W

t~

tb ~,~e

e

Z

tb ~,~

0~

bb~

If the mass split is low (as suggested) this would lead to very soft b-jets and missing pT.

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LEP and CDF/D0 Results

CDF/D0 – measurement at high masses but still relatively hard jets (due to triggers) which are not favored by the dark matter scenario.

LEP – able to measure in the region where the mass difference is only few GeV (?!)

ILC should not be much worse but at higher masses.

Small (meaning tiny) mass splitting is not accessible at ILC.

ILC

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Conclusions

?

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Thank you for attention…