# Presentation: 056164

## PRECISION PHYSICS 

**WITH THE ****ATLAS**** DETECTOR **
**AT THE CERN  ****LHC**
**Theodota Lagouri**
**MPI, Munich/CERN **

-   SM precise  measurements (examples) :
-      -- W mass
-      -- Top mass
-      -- Triple Gauge Couplings
-   SM Higgs physics :
-    -- Discovery potential
-      -- Measurements of parameters
-   B physics:
-      -- CP-violation and aspects of CKM matrix
-      -- Rare B decays

## Large Hadron Collider

- _14 TeV pp Interactions_
- Most recent machine schedule:
- -- 2006: commissioning
- -- 2006: Pilot run
- -- 2006-7: physics run with ~10 fb-1
- ® 2006 run: understand/calibrate the (complex) LHC detectors, using ~ 10 million W ® l v, 2 million Z ® ll events, 1 million tt events
- ® 2006-7 run: ready for “discovery physics”
- Luminosity:
- L peak < 1033 cm-2 s-1  (“low L”)
- L peak <1034 cm-2 s-1 (“high L”)
-                  100 fb-1 per year
- High L: ~ 20 soft interactions per crossing (pile-up)

- ® LHC is  a B-factory, top factory, W/Z factory,
-     Higgs factory, SUSY factory, etc.

- ® Mass reach:  up to » 5 TeV
- ® Precision measurements  limited  by systematics
- (mainly performed at low L ®  ‘clean’ environment)

## LHC  statistics

- Production  rates per experiment  at  1033 cm-2 s-1

##  Lepton measurement:   pT » GeV ® 5 TeV

- MAGNET	Air-core toroids + solenoid in inner cavity 		Calorimeters outside field, 4 magnets
- TRACKER	Si pixel + strips, TRD ® particle ID, 		B=2T, s/pT ~ 5x10-4 pT + 0.01
- EM CALO	Pb- liquid argon, s/E ~ 10%/(E)1/2 , 		uniform longitudinal segmentation
- HAD CALO	Fe-scintillator + Cu-liquid argon (10 l ), 		 s/E ~ 50% /(E)1/2+ 0.03
- MUON		Air ® s/pT ~ 7% at 1 TeV standalone

-  Lepton measurement:   pT » GeV ® 5 TeV
-     ( b ® lX, W’/Z’)

-  Mass resolution  (m ~ 100 GeV) :
-         » 1 %                (H ® gg, 4l)
-         » 10 %              (W ® jj, H  ® bb)

-  Calorimeter coverage : |h| < 5
-    (ETmiss,  forward jet tag for strongly interacting  Higgs)

- Particle identification :
-   eb » 50 %         Rj » 100      (H ® bb, SUSY)
-   et » 50 %         Rj » 100       (A/H ® tt)
-   eg » 80 %         Rj > 103        (H ® gg)
-   ee > 50 %         Rj > 105

-  e/jet ~ 10-3    Ös = 2 TeV
-  e/jet ~ 10-5      Ös = 14 TeV

## Three crucial parameters for precise measurements

## W   mass 

- Year 2006 :    DmW < 30 MeV      LEP2+Tevatron

- Motivation to improve:

- DmW » 0.7  ́ 10-2 Dmtop        to get similar errors

- Dmtop ~ 1.5  GeV   (LHC)    requires

- DmW » 15 MeV

- -- constrains mH  to 25%
- -- if/when Higgs found: check consistency of theory

- Expect  ~ 60 million
- well  measured
- W® ln (l = e, m)
-  for 10 fb-1
- (~ 10 times  larger
- cross-section than at
- Tevatron)

-  Main asset at LHC:  large statistics of  Z ® ll (6 million for 10 fb-1)
-  Most serious challenge: lepton scale must be known to  ~ 0.02%.
-   Should be possible because Z-mass  very close

- ®  hope to measure   mW  to  ± 15  MeV at LHC

## Top mass

-  Year 2006 :        Dmtop » 2-3 GeV         (Tevatron)

-  Best  channel:  t ® Wb ® lnb    t ® Wb ® jjb

- limited  by
- knowledge
- of physics

- Other methods  (high-pT top, 2l and 3l channels): similar
- error  but  different  systematics

- jet scale
- known to 1%

-   10 fb-1
- 1 experiment

## Triple Gauge couplings

-  Probe non-Abelian structure of SU(2) x U(1)
-    and sensitive to New Physics
-   Year 2006 :  g1Z, lg, kg, lZ, kZ  known to  £ 10-2
-    from LEP2+Tevatron
-   Some anomalous contributions  ( l-type)  increase with s
-   ® high sensitivity at LHC
-  Sensitivity from :
-   -- cross-section (mainly l-type) and  pT  measurements
-   -- angular distributions   (mainly k-type)

- Wg ® ln g
- WZ ® ln ll

- 30 fb-1
- 1 experiment

## Higgs Physics

**Higgs Physics**
- _SM  Higgs  production_

## Main search channels at LHC 

- Detector performance is crucial: b-tag, l/g  E-resolution,
- g /j separation, ETmiss resolution,  etc.
- Higgs sector used as benchmark for detector optimisation

## Overall discovery potential for SM Higgs

- _Overall discovery potential for SM Higgs_

- 100 fb-1 : ~ 4 years  of  LHC  operation

## Measurement of the Higgs mass

- No theoretical error
- e.g. mass shift for
- large GH  (interference
- resonant/non-resonant
- production)

- Dominant systematic
- uncertainty: g /l  E scale.
- Assumed     1‰
- Goal            0.2‰

- ATLAS 300 fb-1

- Precision 0.1% - 1%

## Measurements  of  couplings (few preliminary examples)

-    measure ratios of rates for different channels
-     ® cross-section, L,  G  cancel ® get ratios of couplings
-     ® many constraints of theory
-   Errors: statistics (dominant), systematics on background
-     subtraction in some channels

- _From gg-fusion :_

- ATLAS
- 300 fb -1

- 80 £ mH £120 GeV
- precision  ~25%

## Measurement of  SM Higgs width

-    _Direct  measurement_ :  from width of reconstructed mass peak
-    for mH > 200 GeV (GH > Gdetector   in  SM)

- Statistics and systematic
- errors included

- Systematics: luminosity (~5%),
- cross-section (~5%),
- background subtraction (< 5%),
- radiative Z ® llg (1.5%)

- ATLAS
- 300 fb-1

- _Measurement of the Higgs production rates (__s×__BR)_

- Dominant errors: statistics, luminosity, background systematics

- ATLAS 300 fb-1

- Precision 6%
- 300 GeV < mH < 700 GeV

- Typical precision: 7%(12%)
- 120 GeV < mH < 600 GeV

## ATLAS B physics potential 

- CKM Unitarity Triangle

*V*td*V*tb* + *V*cd*V*cb* + *V*ud*V*ub* = 0

- g

- a

- b

*μ **V*ub*

*μ **V*cb

*μ **V*td

- arg *V*cb = 0, arg *V*ub = - g, arg *V*td = - b

- Why B physics around CKM?

- Check CKM matrix description of CPV
- Measurement of fundamental SM parameters (Þare theoretical « predictions » of CKM angles and magnitudes correct? Hints of underlying theory)
- Overconstrained system (triangle closure and **#** measurements of same quantity):
- Check of assumptions in calculations in B systems
- Look for deviations from new physics

## B0®J/y K0s:sin(2b)

- Gold-plated mode: clean both experimentally and theoretically
- Asymmetry:
- A(t)= _N(B__0__®__J/__y__ K__0__s __)-N(B__0__®__J/__y__ K__0__s__)__
-           N(B0®J/y K0s )+N(B0®J/y K0s)
- A(t)=Adir cos(Dmt)+Amix sin(Dmt)
- Adir~0, Amix=-sin(2b)

- ...J/y K0s: results

- After one year
- All combined d(sin2b)=0.017
- All combined three years: d(sin2b)=0.01

## B0s®J/yf

- Yet another golden decay-mode:
- Trigger and reconstruction similar to J/y K0s
- Fast B0S oscillation to be resolved
- Theoretically clean but complex:
- Final state is a mixture of CP eigenstates which can be resolved with angular analysis:

- CP Violation weak phase in Bs0®J/yf

- The xs-fs region allowed by Standard Model (SM),  Left-right symmetric
- models (NP-LR), Iso-singlet down quark mixing Model (NP-DQ) and
- the region of experimental sensitivity of ATLAS.

## Rare decays potential

- FCNC decays b® s, b® d occur only at loop level in SM ®
- Br < O(10-5). Sensitive to new physics.
- B®mm can be seen before LHC only if drastically enhanced comparing to SM.
- Leptonic:
- B0s®m+m- (Br SM~4.10-9), B0d®m+m- (Br SM~1.10-10) (Tevatron experimental limit <10-6)
- Tiny branching ratios: ideal place to search for new physics at LHC
- Self triggering : can be looked for at high luminosity as well
- Reconstruction uses vertexing and isolation
- 4.7 sigma B0s observation after 3 years at low luminosity and one at high luminosity
- Some B0d events but difficult since mass resolution is 70 MeV
- ÞOn going work especially at high luminosity

- Rare decays, conclusions

- Improve and increase statistics seen at e+e- factories or Tevatron.
- Simulations showed that ATLAS is capable to measure at high
- luminosity 1034 cm2s-1 and substantially improve results.
- Measure branching ratio of B0s®m+m- which is of order Br < O (10-9).
- Perform a high sensitivity search  for B0d  ®m+m-

## Conclusions

-   Great ATLAS-LHC potential  for precise measurements
-     (keywords: huge statistics, excellent  detector), e.g.:
-      -- mW  to » 15 MeV
-      -- TGC to 10-3 -10-2
-      -- many measurements in top sector (precision  few %)
-      -- B-physics, Higgs, SUSY, Exotics, etc.
-   ® in many cases improve on  precision from previous machines

-  **Higgs physics**
- SM  Higgs:
-   --   can be discovered  (3 5s) over  full mass range with  10 fb-1
-         (~7 months  at  2x10 33) provided detector is well understood
-   --  with  present machine schedule this observation will be
-        possible  end 2006/ beg 2007

-  Precise measurements of  Higgs parameters possible:
-     -- masses to 0.1% - 1%
-     -- width to  10-20%
-     -- coupling ratios to  ~10-20%
-    ®  _many constraints on  theory  and  first  systematic_
-          _investigation  of  Higgs boson properties _
- Detector  performance is crucial

## B physics

- General purpose LHC central detector ATLAS is well equipped for rigorous B-physics program
- In CP violation the main emphasis will be on the underlying mechanisms and evidence of new physics. ATLAS is especially precise in measurement of b. Appart from e+e- B factory ‘benchmark modes’ the LHC ‘gold-plated’ mode Bs0 ® J/Y f has been  studied
- Rare decays with 2 muons have the most favourable experimental signature allowing to measure also at 1034 cm2s-1. Bs0 ®m+m - with a SM ratio 10-9 will be seen already after one year.

- Large number of accessible channels demonstrates sensitivity
- of ATLAS to large variety of signatures and therefore
- their potential  also  for other scenarios and for
- new / unexpected physics

- Higgs sector  has  been  challenging benchmark for LHC detector
- design and performance optimisation : g / e / m / t energy resolution
- and  identification, ETmiss resolution, b-tagging, multi-jet mass
- spectroscopy, forward jet tag, etc.