# Presentation: 215438

## Inclusive and semi-inclusive DIS    results from COMPASS

**Helena Santos**
** ****LIP - Lisboa**
**On Behalf of the COMPASS Collaboration**

**Parity-Violating Spin Asymmetries at RHIC**

** ****The nucleon spin **
** ****The COMPASS experiment**
** ****Longitudinal spin structure functions**
** ****Valence quark polarisations**

## The Nucleon Spin

**naïve parton model:****         ******** ****= ********u****+********d**** = 1**

**EMC (1988):**

**gluons, sea  and *****c***** quarks are important**

**complete description:         orbital angular momenta**

****** ****= 0.12 ******** 0.09 ******** 0.14      ********s****+********s ****= ****–0.14 ******** 0.03**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## Deep Inelastic Scattering

- Q2 = –q2 = (p – p’)2
-  = E – E’
- x = Q2/2M
- y = /E
- z = E*h*/

****

**Deep Inelastic Scattering**

****(E**,p**)

******’**(E’**,p’**)

**N**(M**,P**)

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## Polarised Deep Inelastic Scattering

**photon-nucleon asymmetry**

******q(x) = q(x)****+**** ****– ****q(x)****–**
**q(x) = q(x)****+**** ****+ q(x)****–**
**+: **quark** ******** **nucleon
**–****: **quark** ******** **nucleon

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## The COMPASS Experiment at the CERN-SPS

**N**

**SPS**

**LHC**

**L****uminosity:  ~5 ****.**** 10****32**** cm****-2****  s****-1**
**B****eam intensity: 2********10****8**** μ****+****/spill (4.8s/16.****8****s) **
**B****eam momentum: 160 GeV/*****c***

**The COMPASS Experiment at the CERN-SPS**

**230 physicists from 12 countries**

**M2**

**CO****mmon**** ****M****uon and**** ****P****roton ****A****pparatus for**** **** ****S****tructure and ****S****pectroscopy**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## The COMPASS Spectrometer

- 50 m

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## The Target System

**Two**** 60 cm long target cells with **_**opposite polarisation**_
**Target material: ****6****LiD**
**Polarisation ~ 50%**
**Solenoid field: 2.5 T**
**3****He/****4****He: T****min**** ~ 50mK**
**Field reversal every 8h**
**In 2006:**
**New solenoid with 180 mrad acceptance**
**Three target cells (better to face systematics)**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## Inclusive Asymmetry, Q2 < 1 (GeV/c)2

** ****A****1****d ****asymmetry compatible with 0 at low x range ****(0.0005<x<0.02)**
** ****At low x A****1****d**** has been measured only by COMPASS and SMC**
** ****Systematic errors are mainly due to false asymmetries**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## The g1(x) Structure Function 

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

**F****2****(x) taken from SMC param. ****(SMC + JKBB: B. Adeva *****et al*****      PRD60 ****(1999) 072004; Erratum-ibid.D62:079902,2000****)**

**x > 0.12 SLAC                    ****(PLB250 (1990) 193; B52(1999)1994)**
**0.003 < x < 0.12 NMC                   ****(R param. unpublished)**
**x < 0.003 ZEUS ****(EPJ7 (1999) 609)**

**Knowledge of g****1**** at low Q****2**** is needed to test non-perturbative models: Regge and (G)VDM**

## Inclusive DIS Asymmetry

** ****A****1**** compatible with 0 for x < 0.05**
** ****Large asymmetry at large x **
** ****Systematic errors: Multiplicative ******** ******** ******** 0.10*****A  *****(********P****B****, ********P****T****, ********f and ********D)**
**	****	****     ****Additive ******** rad. corrections ******** 10****-4**** ****— 10****-3****;**** **** *****A******false***** < 0.4*********A******stat***

**COMPASS       2002, 03 & 04 data **
**89 x 10****6**** events**

**Q****2**** > 1 (GeV/c)****2**
**0.004 < x < 0.7**
**0.1 < y < 0.9**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## Inclusive DIS Asymmetry

** ****Good agreement with previous experiments **
** ****Improved significantly statistics at low x **
** ****No tendency towards negative values at x < 0.03**

**Q****2**** > 1 (GeV/c)****2**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## The g1(x) Structure Function 

**The g****1****(x) Structure Function **

**d**

**Q****2**** > 1 (GeV/c)****2**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## QCD Analyses

**DGLAP equations: **

**Input parameterisations (x****–****dependence at a fixed Q****0****2****): **

-  = u + d + s,    q3 = u - d,       q8 = u + d - 2s

**Minimization routine:**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## QCD Fits

**Data well described by two solutions: ********G > 0 and ********G < 0**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## Towards Structure Functions

**Towards Structure Functions**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## Towards Structure Functions

**Towards Structure Functions**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## Polarised Parton Distributions

** ****Very small sensitivity of x(********q+********q) to x********G**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## QCD Fits Results

**Quark polarisation:**

|  | G > 0 | G < 0 |
| --- | --- | --- |
|  | 0.27 ± 0.01 | 0.32 ± 0.01 |

**Gluon polarisation (****indirect determination via DGLAP****):**

** ****(world data)**

- (error  factor 2 larger without COMPASS)

** ****Solutions with ********G**** > 0:**
** ****Solutions with ********G**** < 0:**

**|********G****| ******** 0.2 - 0.3**

_**Phys. Lett. B 647 (2007) 8**_

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## Gluon Polarisation G/G 

**Comparison between direct measurement of gluon polarisation and COMPASS NLO QCD fits to g****1**
** ****Unpolarised G(x) from MRST**
** ****Bands correspond to statistical errors of ********G**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## First Moment of g1

**â****0**** is interpreted as the fraction of the nucleon spin carried by the quarks,       ******** = ********u + ********d + ********s**

**First Moment of g****1**

**(COMPASS data only)**

-  (from Y. Goto *et al*., PRD62 (2000) 034017:  a8 = 0.585 ± 0.025)

**   ****extrapolating to Q****2**** ******** ******

_**Phys. Lett. B 647 (2007) 8**_

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## Semi-inclusive asymmetries

** ****COMPASS**** kinematic domain:  inclusive DIS     +     0.2 < z < 0.85**
** ****Statistics:****  N****+**** = 30x10****6****,     N****–**** = 25x10****6****,     corr(N****+****,N****–****) ******** 20%**
** ****Systematic errors:**** Multiplicative ******** ******** ******** 0.08*****A  *****(********P****B****, ********P****T****, ********f and ********D)**
**              ****	****	****     ****Additive: rad. corrections ******** 10****-5**** ****— 10****-4****;****    *****A******false***** < 0.52*********A******stat****** ***

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## Difference asymmetry

** ****In LO QCD FF do cancel out in A****+****–****. ****For a deuteron target:**

** ****The contribution of ****sea quarks**** to the nucleon spin can be obtained by combining the matrix elements a****0**** and a****8**** and the ****integral**** **

** ****To disentangle between **_**symmetric**_** and **_**asymmetric**_** ****sea**** scenarios                          ****	********v ****< 2|********s+********s| is needed**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## Difference asymmetry

**Difference asymmetry**

**Compass 2002, 03 &**** ****04 data**

** ****The measured x range is ****0.006 < x < 0.7****, as the precision at smaller x is too low. (****The statistical error is inversily proportional to *****N******h+****** ******– N******h–*****)**
** ****For the acceptance studies full chain of MC simulation (spectrometer + same cuts as for data) with default LEPTO settings was performed**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## Valence quark polarisations

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

- (*D* = 0.05  0.01)

** ****Much better precision**

** ****Unpol. sea contribution  to *****F*****2**** vanishes for x > 0.3**
**  ****|********u+********d| < u+d**

** ****All points evolve to Q****0****2**** = 10 (GeV/c)****2**** accordingly to DNS parameterisation         ****	****(D. De Florian, G.A. Navarro and R. Sassot, Phys. Rev. D71 (2005) 094018) **
** ****LO DNS analysis, based on KKP param. of FF, includes:                                                  ****	****All DIS g****1**** prior to ****COMPASS ****2004 data;                                                 ****	****	****All SIDIS data from ****SMC**** and ****HERMES ****(********u = ********d = ********s = 0 for x > 0.3)**
** ****Unpolarised MRST 2004 LO PDFs have been used **

## Comparison with other experiments

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## Comparison with other experiments

**Comparison with other experiments**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## Comparison with other experiments

**Comparison with other experiments**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## Comparison with other experiments

**Comparison with other experiments**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

**DNS parameterisation predicts successfully ****COMPASS**** ****SIDIS**** data**

## Estimate for the first moments (LO)

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

** ****Contribution from the unmeasured 0.7 < x < 1 region is 0.004 (DNS fit)**
** ****SU(3) symmetric sea was assumed in SMC**
** ****The estimated ********v**** (SIDIS + DIS) is 2.5*********stat*****  away from the symmetric sea scenario**

## Conclusions

**Conclusions**

** ****QCD fits to world data give for **_**quark and gluon contributions**_**:**

**|********G| ******** 0.2 ****—**** 0.3**

** ********u****v**** + ********d****v**** have been extracted from difference asymmetry approach**
** ****Increase of the precision at small x by a factor of ~6 as compared to SMC**
** ****DNS parameterisation predicts successfully COMPASS SIDIS data**
** ****SU(3) symmetric sea scenario is disfavoured**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)

## The Longitudinal Photon-Deuteron Asymmetry

**The Longitudinal Photon-Deuteron Asymmetry**

****** ****is small in the COMPASS kinematic range, as well as A****2**

**N  ******** Number of detected events**
**P****B**** ******** Beam polarisation ~80%  (********5%)**
**P****T**** ******** Target polarisation ~50% (********5%)**
**f    ******** Dilution factor ~40%         (********2%)**
**D  ******** Depolarisation factor        (********3%)**

**(d ******** deuteron)**

****** ******** ****1****st**** cell (polarised anti-paralell to beam), ******** ******** 2nd cell (polarised parallel to beam)**

-  Parity-Violating Spin Asymmetries at RHIC,  April, 26th – 27th  2007                                                                                                         H. Santos  (helena@lip.pt)