SLHC Physics and B tagging

SLHC Physics and B tagging

SLHC Physics and B tagging

Joe IncandelaUniversity of California, Santa Barbara

10/12/06

Joe IncandelaUniversity of California, Santa Barbara

10/12/06

SLHC Physics and B-tagging : FNAL Pixel Workshop, October 12, 2006 J. Incandela (UCSB) 2

Outline• The SLHC Physics Argument (cf. Eur. Phys. J. C39 (2005)

293)• The physics case as from the viewpoint of the tracker

• Tracking and tagging at high luminosity• ATLAS B tag study• CMS Heavy Ions: • Lepton track triggers

• B tagging in CMS @ LHC. • B tagging in CMS @ SLHC:

• Does it become less important?• What would be needed to make it work as well as at LHC?

• Some remarks


Disclaimer• This talk was put together on short notice and mostly in

an airplane… • Not intended as a final word on anything…• In the spirit of a workshop – it is meant mostly as a point

of departure for more work and discussion


SLHC Physics

P.Allport @Hiroshima ‘06


As viewed from the tracker

• Can divide SLHC physics into several relevant categories1. Relies on tracking and/or b tagging

2. Requires absence of a track and/or b tag

3. Requires both 1 and 2• E.g. Vector boson resonance

Need to identify high energy leptons

Veto events with b tags to help eliminate tt background

If no light higgs seen, this will be a major emphasis of

SLHC


Leptons are top priority

• Multi-boson couplings & Higgs pair production• Both rely upon efficient detection of leptons• Leptons should be the 1st priority of SLHC tracking

All new ground. up to 6 l final

states!

Only sensitive if H above threshold to decay to VB pairs-

170 ≤ MH ≤ 200


The case for track triggers @ L1• From Gianotti et al. Eur. Phys. J. C 39, 293-333(2005)

• Larger event size due to higher occupancy means that 100 kHz L1 rate will probably need to be maintained…”such a strategy… implies raising transverse momentum thresholds on candidate electrons, photons, muons, etc. and using less inclusive triggers…”


Higgs• Discovered Higgs at LHC• At SLHC interested in

rare decays, couplings to fermions and bosons.

• HZ, H +- …• H and WH lComparison tells us about Htt coupling since gg H proceeds via top loop

• Or if SUSY seen, we would want to extend our reach for a 2nd and heavier SUSY higgs• Need b/tau tagging.


Degradation of b tagging & Electron id

• No studies by CMS yet. • ATLAS has studied

current detector w/full simulations• Mistagging rises x3 to x8

for fixed 50% b tag rate• Rate of jets faking

electrons at fixed 80% electron efficiency nearly doubles.


B tagging in CMS• Very similar to Tevatron tagging

• Track tags • Jet probability• Number of tracks (2 or 3) above some impact parameter

significance• Vertex tagging

• Effectively similar in many ways to CDF SECVTX algorithm but developed in a much more intelligent way*

* ( I can say this since I was one of the original developers of the CDF algorithm)


Inputs to Combined B tag Algorithm

• Jet Reconstruction: Iterative Cone Algorithm with cone size = 0.5

•

Track Reconstruction: CombinatorialTrackFinder• ≥ 8 hits in total (pixel + strips) and ≥ 2 pixel hits• pt > 1 GeV/c• χ2/dof < 10

• dxy < 2 mm (transverse impact parameter)

• Vertex Reconstruction:• Primary Vertex: Global Reconstruction

• PVFPrimaryVertexFinder with reduced pt=0.7 GeV/c for tracks

• See CMS AN 2005/043 (C.Piasecki, C.Weiser et al.)• Cone based association of tracks to jets: ΔR < 0.3


Two Definitions1) “Physics” Definition:

• Match reconstructed jets to “initial” partons from the primary physics process (within ΔR < 0.3 of reconstructed jet cone)• e.g. For tt the initial

partons are: 2 b jets from top decays 2 non-b jets per hadronic W decay & no initial gluon jets

• jets from radiation are not matched with full efficiency

• Gluon jets splitting to c- or b- quarks are labeled “gluon”

2) “Algorithmic” Definition:• Try to find the parton that

most likely determines the properties of the jet and assign that flavour as true flavour

• here, the “final state” partons (after showering, radiation) are analysed (also within ΔR < 0.3 of reconstructed jet cone)

• jets from radiation are matched with full efficiency

• if there is a b/c within the jet cone: label it as b/c otherwise: assign flavour of the hardest parton


The Algorithm• Inclusive vertex reconstruction in jets using the “Trimmed

Kalman Vertex Finder”;• select secondary vertex candidates:

• 100 μm < Lxy > < 2.5 cm

• Significance (Lxy/σ) > 3

• Invariant Mass of tracks in vertex candidate < 6.5 GeV• Reject if vertex has two oppositely charged particles with invariant

mass within 50 MeV of K0 mass

• 3 Categories: depend on result of inclusive vertex reconstruction:1. “RecoVertex”: at least one accepted SV candidate found

2. “PseudoVertex” : built from tracks incompatible with the primary vertex

(d/ > 2), if at least two such tracks are present

3. “NoVertex”: the rest


Vertex Categories

bcuds

QCD 50-80|η| < 2.4


Input Variables I

Enter for all categories into the final discriminatorFurthermore: -sort tracks in decreasing order of IP significance -compute mass for tracks -look at IP significance of track pushing the mass above threshold related to charm hadron mass (here: 1.5 GeV)

“RecoVertex” “PseudoVertex” “NoVertex”

bcuds

QCD 50-80|η| < 2.4

Lifetime signed 2D Track Impact Parameter Significances

C.Weiser A Combined SV Based B-Tagging Algorithm in CMS CMS Phys. Meeting 6/12/2005 P 16

Input Variables IIAdditional secondary vertex related variables for category 1 (“RecoVertex”)

bcuds

inv. mass of charged particles at SV

multiplicity of charged particles at SV

PV-SV σPV-SV

QCD 50-80|η| < 2.4


Input Variables IIIAdditional secondary vertex related variables for category 1 (“RecoVertex”)

En

ters

for

n t

racks

bcuds

fractional chargedenergy at SV

IP sign. of first trackabove charm mass

rapidities ofcharged particlesat SV QCD 50-80

|η| < 2.4


Input Variables IVAdditional secondary vertex related variables for category 2 (“PseudoVertex”)

bcuds

inv. mass of charged particles at SV

multiplicity of charged particles at SV fractional charged

energy at SV

rapidities ofcharged particlesat SV

IP sign. of first trackabove charm mass

QCD 50-80|η| < 2.4


bcudsg

QCD 50-80|η| < 2.4

bcudsg

Plots have been obtained by scanning the cut on the discriminator

Final Discriminator II


QCD 50-80 GeV


Misidentification efficiencies for fixed b-tagging efficiency of 50%

|η| < 2.4 QCD 50-80

pt

non

b-je

t ef

ficie

ncy

loss of tracks in this bin!

● uds * g ▲ c

η

Dependence On PT and η


Heavy Ions as a proxy for pp@SLHC


HI AlgorithmDefault pp algorithm with following modifications:

1) Trajectory Seed Generation• Three pixel hit combinations (Pixel triplets)• Primary vertex constraint

2) Trajectory Building• Includes all material effects

• multiple scattering • energy loss

• Special error assignment to merged hits

3) Trajectory cleaning• Allow only one track per trajectory seed: best 2

4) Trajectory Smoothing • Final fit with split stereo layers


Acceptance

Require 8 strip layers (~12 hits) and 3 pixel layers.Geometrical acceptance ~80%


Track quality Cuts

• More than 12 hits on track (stereo layer => 2 hits)• Require fit probability > 0.01• (Cut on compatibility with primary vertex)

Good reconstruction


Fakes

• Fakes substantially higher than at LHC, as seen by ATLAS.

nhit > 12 nhit > 12pchi2 > 0.01

• Efficiencyo Fake Rate

• Efficiencyo Fake Rate

looser


Trajectory Building

• Number of candidates drops fast as you move to larger radius even though the occupancy does not fall as quickly.• Track gets more refined and so road narrows…


High efficiency setting

Algorithmic Efficiency and Fake Rate vs Algorithmic Efficiency and Fake Rate

vs


Algorithmic Efficiency and Fake Rate vs

Low Fake Rate Setting


CMS Subtraction

Would all strips

continue to need to be

read out?


Go back to that occupancy plot…• Mismatch at system

boundaries• Layer 4 appears pretty

useless here.• 3 cm strip would cut

occupancy to under 10%

• Pixel layer would likely ice the cake

• Layers 5-13• Shorter strips but

fewer layers to compensate for material? A factor of 3 reduction in strip length would do a lot for this plot.


ATLAS Granularity Guide

Using mainly strips, they’d

get substantially better

granularity than CMS has now


But granularity can be wasted…• Fine Granularity is necessary but not sufficient…

• It is wasted if there is substantial multiple scattering, radiation, and secondary particles generated in material interactions.

• Material can set an effective granularity if one is not careful• Material reduction throughout the tracker would enhance our

effective granularity now, without changing anything else.


Multiple Scattering Now

Cucciarelli et al. CMS Note 2006/026


Well this is a workshop…• I don’t have answers… want to 1st frame the questions correctly

• Main points:• Leptons 1st displaced tracks 2nd on priority list (just in case push comes to

shove…)• Granularity can be improved substantially without necessarily using

pixels - but it will be meaningless if the material budget is not reduced.

• Some thoughts• Some technologies could run warm (e.g. n-in-p discussed in Mara Bruzzi’s talk

tuesday) … This might allow us to eliminate much of the cooling related mass in the tracking volume.

• Super thin is in. We need to think more like e+e- people regarding our material budgets

• Recognizing that material can alter the effective granularity of a tracker, we should carefully consider the possible benefits of a reduction in the number of layers !

• We need to do some serious studies of these issues• Triggering was not covered in this talk, but is important, particularly for

leptons


Tangent-Point Reconstruction

α

J.Jones Imperial College London (Perugia Workshop) 37

More stuff


construction costs• Upgrade cost ~

200MCHF• Broad brush estimate

reported to CMS MB/CB & DG• ~60-70% tracker related• plus staff costs (significant)

Inner Tracker 25-30 MCHF

Outer Tracker 90 MCHF

Level 1 Trigger 15 MCHF

DAQ 10 MCHF

Other Front Ends

5-10 MCHF

extra costs 10ns/15ns

20-30 MCHF

Infrastructure 15 MCHF

G. Hall, Imperial CollegeCMS Tracker Meetings, CERN - August 2006


Roadmap from previous workshop

• Installation of modest system at t = to + 5y may be possible• Lower cost and risk• Allows trial of components or devices, which may still evolve• May be possible to react to LHC conditions

• machine, experiment or even discoveries• An evolutionary approach to replacing full tracker?• Ideas are still to gel but must do so soon

• CMS proposes common EoI (2006), and LoI (mid-2007)• R&D proposals to be evaluated by CMS, and approved/encouraged

2006 2007 2008 2009 2010 2011 2012 2013 2014 2015

New Layers Concept New ROC/New Sensor Fabricate Install

Full Tracker Monte Carlo Concept New ROC/New Sensor Fabricate

G. Hall, Imperial College


You can’t always get what you want…• But how much increase in granularity is actually

needed?• Present microstrip occupancies are 0.5% - 2.8% in barrel

CMS NOTE 2002/047



Potential synergy with ATLAS• ATLAS & CMS have exchanged speakers at several workshops

• Initial phases of LHC R&D were common• Although many similarities, also important points of divergence

• eg sensor design, electrical & optical interfaces, analogue/digital, DAQ design,..

• Possible common efforts• Sharing ASIC processing runs (in CMS & CERN done well for 0.25µm CMOS)

• Advantageous to share circuits, evaluate technology and adopt common standards

• Share development of common SLHC systems• Optical links and Timing-Trigger-Control system are prominent items• Common effort on power provision - eg DC-DC conversion?

• Dialogue with machine• Agree clock speed, verify current systems, information about machine

operation• Special tooling

• removal and installation of irradiated systems in irradiated environment • Information exchange via regular meetings

• Annual LECC workshops are one common forum for electronic R&D• Comparison of cooling system performance might be profitable



Cooling costs• Using power has heavy

material cost• For present pixel

system• Power in ~4%• Power out ~29%

• For microstrips• Cables ≈ Cooling• Cables + Cooling +

Support ≈ 2x (Sensors+ Electronics)


Noise & granularity• Leakage current shot noise also determines element size

• Noise scales ~ (area, time, fluence, shaping time,...)1/2• How much can be gained by cooling? (ATLAS discuss -35°C)

• Ileak will be more significant power burden so must be contained

• thermal runaway is increased danger• Is required lifetime again 10 years?


Seeds


Performance of the Track Reconstruction

• Match Reconstructed tracks to MC input on a hit by hit basis.

(Event sample: dn/dy ~3000 + one 100GeV Jet/Event)

Momentum Resolution Transverse Impact Parameter Resolution

Longitudinal ImpactParameter Resolution

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SLHC Physics and B tagging

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